In the meantime, here is some good reading for everyone:
Mission Statement: 
Since 2.1 released, there has been a lot of debate over what weapon types are most effective.
Also there's vastly differing opinions on what passives and  even  skill runes, should be used. 
Once and for all, I wanted to really sit down and figure  out  all of these things.
  If anyone wants further proof of any of the points made below, utilize these links:
All of this the data comes
 RIOKOU'S personal  calculations:
http://i.imgur.com/ZKQ9hdF.jpg
See this post for a summary and more discussion: 
M6 Theorycrafting - The best weapons, skills, and passives
  
For the most up-to-date weapon rankings using Best-in-Slot stats:
          
  
Assumptions:
- The arrows that Frost Arrow splits into do the same damage as the initial hit (330%) 
 
- Frost Arrow always hits as many targets as are in range, up to the max it can hit (11 targets) 
 
- The rockets fired by Spitfire Turret follow "normal" breakpoints rather than the odd ones the sentry's spenders have. 
 
Other notes:
- This post is focused on the Cold/Fire build using Cluster Arrow  (Maelstrom), Elemental Arrow (Frost Arrow), Multishot (Arsenal), and  Sentry (Spitfire Turret), along with the Ballistics passive. As far as I  can tell, this is the best build right now. 
 
- This post also tends to focus on the 2.16, 2.84, and 4.15 breakpoints, which are the ones used when Tasker and Theo is used. Utilize the spreadsheet above to see the effects on non-TnT builds. 
 
- Whenever I say "overall damage" I am referring to the total damage  done by all of your sentries' attacks in a given period (but not your  animals/pets). 
 
- Figuring out "real" DPS is hard to do because the number and density  of targets can vary wildly. Luckily, rockets (which actually make up as  much as 60% of your damage) are easy to simulate since they are  single-target and seek out enemies. This helps keep variability down.  There were still some things to make up, like how many targets Cluster  Arrow and Multishot will usually hit (3? 5? 10?), but those kinds of  decisions did not make too much difference in the end. 
 
- Whenever the AoE effects of Multishot and Cluster Arrow are  discussed, I am assuming these effects hit every target in the scenario  with every shot, whether I am taking about 1 target or 5 or 10. 
 
- I used REHWYN'S work on sentry skill cast rates found [HERE] in these calculations 
 
- How rockets work: Whenever a skill fires some rockets, those rockets  seek out enemies anywhere in range. In each set of rockets fired, each  rocket will hit its own enemy. This means if a skill fires 5 rockets  (e.g. Maelstrom) and there is only 1 enemy in range, 4 rockets will hit nothing. 
 
   
CA = Cluster Arrow (Maelstrom)
MS = Multishot (Arsenal)
EA = Elemental Arrow (Frost Arrow)
SA = Steady Aim
CE = Custom Engineering
   
  Table of Contents:
 
  1. True Sentry Skill Cast Ratios
  2. Skill Damage: MultiShot vs. Cluster Arrow vs. Elemental Arrow
- WHAT SKILL DAMAGE SHOULD YOU GET ON YOUR GEAR? 
 
    3. Elemental Damage: Cold vs. Fire
  4. Elemental Arrow vs. Impale
  5. Spitfire Turret vs. Polar Station
  6. Steady Aim vs. Custom Engineering
  7. Ballistics Info
  8. The Best Weapons
  9. Kridershot
  10. Additional Info
   
  
  1. True Sentry Skill Cast Ratios
 
  Here are accurate skill cast ratios when using CA/MS/EA. 
This data is derived from 
REHWYN'S work on sentry skill cast rates 
- Originally found [HERE] and improves upon the values found [HERE]. 
 
 
BASIC RATIOS
   [TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
Breakpoint[/TD]
[TD="align: center"]Cluster Arrow[/TD]
[TD="align: center"]Multishot[/TD]
[TD="align: center"]Elemental Arrow[/TD]
[/TR]
[TR]
[TD="align: center"]1.45946[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]1[/TD]
[/TR]
[TR]
[TD="align: center"]1.74194[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]2[/TD]
[/TR]
[TR]
[TD="align: center"]2.16001[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]3[/TD]
[/TR]
[TR]
[TD="align: center"]2.74211[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]9[/TD]
[/TR]
[TR]
[TD="align: center"]4.15386[/TD]
[TD="align: center"]3[/TD]
[TD="align: center"]8[/TD]
[TD="align: center"]23[/TD]
[/TR]
[/TABLE]
Cast Amounts over 30 Seconds
[TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
Breakpoint[/TD]
[TD="align: center"]Cluster Arrow[/TD]
[TD="align: center"]Multishot[/TD]
[TD="align: center"]Elemental Arrow[/TD]
[/TR]
[TR]
[TD="align: center"]1.45946[/TD]
[TD="align: center"]12.5[/TD]
[TD="align: center"]25[/TD]
[TD="align: center"]12.5[/TD]
[/TR]
[TR]
[TD="align: center"]1.74194[/TD]
[TD="align: center"]12[/TD]
[TD="align: center"]24[/TD]
[TD="align: center"]24[/TD]
[/TR]
[TR]
[TD="align: center"]2.16001[/TD]
[TD="align: center"]12.5[/TD]
[TD="align: center"]25[/TD]
[TD="align: center"]37.5[/TD]
[/TR]
[TR]
[TD="align: center"]2.84211[/TD]
[TD="align: center"]12.5[/TD]
[TD="align: center"]31.25[/TD]
[TD="align: center"]56.25[/TD]
[/TR]
[TR]
[TD="align: center"]4.15386[/TD]
[TD="align: center"]13.24[/TD]
[TD="align: center"]35.29[/TD]
[TD="align: center"]101.47[/TD]
[/TR]
[/TABLE]
*Note that Frost Arrow chill uptime is very low at the 1.46 breakpoint  since you get less
 than 1 EA every 2 seconds and the chill effect from  Frost Arrow lasts only 1 second.
    
  2. Skill Damage: MS vs CA vs EA
WHAT SKILL DAMAGE SHOULD YOU GET ON YOUR GEAR?
  Here is a breakdown of damage done by each skill under different  scenarios and breakpoints using +20% Cold Damage in all cases.
These  numbers also arbitrarily limit the AoE portion of Cluster Arrow to a  maximum of 7 targets to account for its relatively small range.
  These values are % weapon damage done by each skill by one sentry over a 30-second period.
T L ; D R :
 [TABLE="class: outer_border, width: 700, align: center"]
[TR]
[TD]
All Breakpoints under 4.15:[/TD]
[TD]
Use +Multishot Damage% on your gear[/TD]
[/TR]
[TR]
[TD]
All Breakpoints above 4.15:[/TD]
[TD]
Use +Elemental Arrow Damage% on your gear[/TD]
[/TR]
[/TABLE]
Multishot Damage% is ALWAYS better than Cluster Arrow
until Elemental Arrow beats both out above 4.15.
  [TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Multishot Damage
[/TD]
[TD="align: center"]24000[/TD]
[TD="align: center"]72000[/TD]
[TD="align: center"]90000[/TD]
[TD="align: center"]135000[/TD]
[/TR]
[TR]
[TD="align: center"]Cluster Arrow Damage
[/TD]
[TD="align: center"]21750[/TD]
[TD="align: center"]65250[/TD]
[TD="align: center"]108750[/TD]
[TD="align: center"]125250[/TD]
[/TR]
[TR]
[TD="align: center"]Elemental Arrow Damage
[/TD]
[TD="align: center"]14850[/TD]
[TD="align: center"]44550[/TD]
[TD="align: center"]74250[/TD]
[TD="align: center"]148500[/TD]
[/TR]
[/TABLE]
*MultiShot wins at low and high numbers of targets hit
2.84 Breakpoint
[TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Multishot Damage
[/TD]
[TD="align: center"]30000[/TD]
[TD="align: center"]90000[/TD]
[TD="align: center"]112500[/TD]
[TD="align: center"]168750[/TD]
[/TR]
[TR]
[TD="align: center"]Cluster Arrow Damage
[/TD]
[TD="align: center"]21750[/TD]
[TD="align: center"]65250[/TD]
[TD="align: center"]108750[/TD]
[TD="align: center"]125250[/TD]
[/TR]
[TR]
[TD="align: center"]Elemental Arrow Damage
[/TD]
[TD="align: center"]22275[/TD]
[TD="align: center"]66825[/TD]
[TD="align: center"]111375[/TD]
[TD="align: center"]222750[/TD]
[/TR]
[/TABLE]
*MultiShot clearly wins at this breakpoint
*Elemental Arrow is good vs. huge mobs but usually won't hit a lot.
4.15 Breakpoint
[TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Multishot Damage
[/TD]
[TD="align: center"]33882[/TD]
[TD="align: center"]101647[/TD]
[TD="align: center"]127059[/TD]
[TD="align: center"]190588[/TD]
[/TR]
[TR]
[TD="align: center"]Cluster Arrow Damage
[/TD]
[TD="align: center"]23029[/TD]
[TD="align: center"]69088[/TD]
[TD="align: center"]115147[/TD]
[TD="align: center"]132618[/TD]
[/TR]
[TR]
[TD="align: center"]Elemental Arrow Damage
[/TD]
[TD="align: center"]40182[/TD]
[TD="align: center"]120547[/TD]
[TD="align: center"]200912[/TD]
[TD="align: center"]401824[/TD]
[/TR]
[/TABLE]
*Elemental Arrow scales VERY well at this breakpoint.
 
The reason MS does so much more damage than CA against 1-3 targets:
- Cluster Arrow fires 5 Rockets for 900% each. 
 
- Multishot fires 6 Rockets for 600% each. 
 
- Multishot fires about twice as often 
 
- Multishot's rockets connect with more enemies than Cluster Arrow's against one to three enemies. 
 
   
3. Elemental Damage: Cold vs Fire
 
  It's safe to say the majority of your damage is 
Cold Damage.  
However, when hitting 1-3 Targets, a majority is 
Fire Damage.
Therefore it's not out of the question to  have some Fire Damage on gear.
  No +Elemental Damage% will be used in these comparisons. 
These numbers also  arbitrarily limit the AoE portion of CA to a max of 7  targets.
- This accounts for its relatively small range of CA.
 
TL;DR: 
Cold Damage is probably best overall, but Fire Damage is a   very close second. 
If you get a piece of gear that is very   good but has Fire Damage% instead of Cold, use it.
Fire Damage is best when hitting 1 or 2 targets.
Exception: If you're gearing for the 4.15   breakpoint, then avoid Fire damage on gear.
The values in charts below represent 
- % weapon damage done
- by each element
 
- by one sentry
 
- over a 30-second period.
 
 
  2.16 Breakpoint
 [TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Cold Damage
[/TD]
[TD="align: center"]30500
*
[/TD]
[TD="align: center"]91500
*
[/TD]
[TD="align: center"]152500
*
[/TD]
[TD="align: center"]228125
*
[/TD]
[/TR]
[TR]
[TD="align: center"]Fire Damage
[/TD]
[TD="align: center"]
42000***
[/TD]
[TD="align: center"]97200
[/TD]
[TD="align: center"]117900
[/TD]
[TD="align: center"]169650
[/TD]
[/TR]
[/TABLE]
*15% MultiShot Damage used with Cold Damage
***Fire has a significant single-target advantage
2.84 Breakpoint
 [TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Cold Damage
[/TD]
[TD="align: center"]36688
*
[/TD]
[TD="align: center"]110063
*
[/TD]
[TD="align: center"]183438
*
[/TD]
[TD="align: center"]290000
*
[/TD]
[/TR]
[TR]
[TD="align: center"]Fire Damage
[/TD]
[TD="align: center"]
52500***
[/TD]
[TD="align: center"]121500
[/TD]
[TD="align: center"]147375
[/TD]
[TD="align: center"]212063
[/TD]
[/TR]
[/TABLE]
*15% MultiShot Damage used with Cold Damage
***Fire has a significant single-target advantage
4.15 Breakpoint
 [TABLE="class: grid, width: 700, align: center"]
[TR]
[TD="align: center"]
NUMBER OF TARGETS:
[/TD]
[TD="align: center"]
1
[/TD]
[TD="align: center"]
3
[/TD]
[TD="align: center"]
5
[/TD]
[TD="align: center"]
10
[/TD]
[/TR]
[TR]
[TD="align: center"]Cold Damage
[/TD]
[TD="align: center"]57699
*
[/TD]
[TD="align: center"]173098
*
[/TD]
[TD="align: center"]288496
*
[/TD]
[TD="align: center"]495596
*
[/TD]
[/TR]
[TR]
[TD="align: center"]Fire Damage
[/TD]
[TD="align: center"]57882
[/TD]
[TD="align: center"]125647[/TD]
[TD="align: center"]151059[/TD]
[TD="align: center"]214588[/TD]
[/TR]
[/TABLE]
*15% MultiShot Damage used with Cold Damage
Cold has a MUCH larger advantage at this breakpoint.
      
  4. Elemental Arrow vs Impale
 
  Some people have proposed using Impale (Overpenetration) in place of  Elemental Arrow (Frost Arrow) due to its superior single-target DPS. It  is true that Impale does great damage against 1 target, however for  Greater Rifts it is not a suitable replacement for Elemental Arrow.
- Against 1 target, using Impale over EA results in a ~15%  improvement in overall damage, but only if someone else in your party  provides constant crowd control. 
 
- Against 2 targets, Impale is about equal to EA in the worst-case  scenario. In the best case scenario (Impale always hits both targets),  it is still about 15% better. 
 
- For anything with more than 2 targets, EA performs better. 
 
  The most important thing to note here is that using Impale in place  of EA means you need to use Sentry (Polar Station) if playing solo in  order to retain the chill aspect of the build to combo with Cull the  Weak and Bane of the Trapped. This is a bad idea because switching from  Spitfire Turret loses you more damage than you gain by using Impale (see  section 3).
  The only way you can get away with using Impale is if you are doing  Ubers or single bosses in a party, and someone else in the party can  provide the crowd-control effect needed to trigger Cull the Weak and  Bane of the Trapped 
so that you can still use Sentry (Spitfire Turret).
  Now that we've determined that Impale is only good against 1-2  targets, that means if you use it you should also take the Chemical  Burns rune, which does superior single-target damage.
  
TL;DR: Use Impale (Chemical Burns) for bossing and maybe Ubers if  you want to, but only if someone else in your party provides constant  crowd-control so you can still use Spitfire Turret. For everything else,  use Elemental Arrow (Frost Arrow).
   
  5. Sentry - Spitfire Turret vs Polar Station
 
  Spitfire Turret does a 
lot of damage, so you shouldn't switch to Polar Station, except 
maybe at the 1.46 and 1.74 breakpoints (due to low Frost Arrow cast rates.)
- Against 5-10 targets, Spitfire Turret does 4-5% of your overall damage. 
 
- Against 1 target, Spitfire Turret does about 20% of your overall  damage. This is because it shoots more often than any other skill, and  also because it only fires one rocket at a time (no rockets are ever  wasted.) 
 
   
  6. Steady Aim vs Custom Engineering
 
  Edit: My numbers here do not factor in the effects of Bane of the Powerful or the Wolf Companion buff. Please see /u/exoscythe's post here for a more realistic comparison of SA and CE.
  Practically all "fights" (i.e. situations where you kill a bunch of  mobs in an area then continue to the next pack) last a minute or so at  most. Because of this, Steady Aim outperforms Custom Engineering  overall.
  
- Assuming you keep Steady Aim active 100% of the time (which you probably don't), Steady Aim outperforms Custom Engineering in fights lasting 84 seconds or less. For 30 second fights, SA gives you 12% more damage than CE does. 
 
- If you have 90% uptime on SA, it is better on fights up to 63 seconds long. For 30 second fights, it gives you 10% more damage than CE does. 
 
- Even with 80% uptime on SA, it is still better on fights up to 50 seconds long. For 30 second fights, it gives you 8% more damage than CE does. 
 
  These numbers also assume that every mob in the fight always gets hit  by every sentry, and that every mob also dies at the same time. In  reality, this is not the case, which translates to SA being even better  than these numbers say.
  On the other hand, it is sometimes possible (and beneficial) to place  your sentries in one area and then continue to pull mobs back to your  sentries rather than move your sentries to the mobs. It could be argued  that this gives CE some extra utility that SA doesn't provide, though I  don't think I would personally choose CE because of this.
  
TL;DR: You should probably pick Steady Aim over Custom Engineering,  except maybe for doing Greater Rift Trials, or if you have trouble  staying away from mobs. See this post for a better comparison of the two abilities.
   
  7. Ballistics Info
 
- Against 1 target, Ballistics gives about a 43% overall damage boost (using all 3 rocket runes) 
 
- Against 5 targets, it's a ~30% boost. 
 
- Against 10 targets, it's an ~18% boost. 
 
  TL;DR: Ballistics is very good
   
  8. The Best Weapons
 
  See this spreadsheet for the most up-to-date weapon rankings using best-in-slot stats.
  For general gearing purposes (i.e. not at the best-in-slot level)  assume all Bows (@2.84) and 2H XBows (@2.16) are essentially equal, with  Etrayu with %Damage pulling ahead (possibly at the cost of some  toughness), and 1H XBows (@2.84) falling a little behind. 
The most important thing is to not waste attack speed if you can avoid it.
  The biggest update in patch 2.1 for M6 was the changes made to "fix"  sentry attack speed. Lower breakpoints still end up being the most  efficient ones on paper, 
however higher breakpoints are  actually better in practice due to the way gearing works. Pretty much  any M6 build will have a quiver, Ring of Royal Grandeur, Tasker and  Theo, and possibly even a Witching Hour. All of these pieces either have  unavoidable or nearly unavoidable Increased Attack Speed stats, which  makes it impossible to hit lower breakpoints without wasting IAS,  especially once you factor in Paragon points. Using higher breakpoints  (2.84 and 4.15) essentially allows you to cash in on this extra IAS for  free. 
   
  9. Kridershot
 
  Kridershot has the potential to be pretty good at the cost of a hit  to your survivability. Kridershot significantly improves your DPS during  the initial ramp-up period when placing your first couple of sentries.
  Note: The following scenarios assume 100% uptime on casting your own  EAs and CAs. In reality, you have to stop to place sentries and dodge  things, so the numbers will be a little lower.
- Worst-case scenario: Fighting 1 target, and over the course of  the fight you have an average of 4 sentries up. In this scenario,  shooting your own EAs and CAs is a 6% damage boost. 
 
- Average-case scenario: Fighting ~8 targets, and over the course  of the fight you have an average of 3 sentries up. Here you get roughly  an 11% boost from Kridershot. 
 
- Best-case scenario: Fighting 11 targets, and over the course of the fight you have an average of 2 sentries up. Kridershot gives you as much as 18% more damage. 
 
  If you have a Kridershot that's rolled well, it's probably a good  idea to use it. Using it also lets you take a more active roll alongside  your sentries, which is a much more fun way to play.
   
  10. Additional Info
 
  
- If you have the option of choosing between Elemental Damage (Cold or  Fire) and Dex on a piece of gear, Elemental Damage is essentially  always better (DPS-wise), unless for whatever reason you have very  little Dex (<7000). Dex however also grants armor, so if you care  about toughness at all it's probably better to have it on your amulet,  where you can roll up to 750 Dex. 
 
   
  
  See this post for a summary and more discussion:
  M6 Theorycrafting - The best weapons, skills, and passives
  See this spreadsheet for the most up-to-date weapon rankings using best-in-slot stats