Questing is 100% the fastest way to level.
Kick's scripts, running 14-16 hour days, can go from 1-80 in 3-4 days. 80-85, in 2-3 days, and 85-90 in about 5 days.
They are very well written, however if you are not something tanky, it could take longer when at the higher levels due to the increased number of elites. At the lower levels, 1-60 takes a very long time as it maintains a quest chain of green and some grey quests, and ideally if power-leveling, you should have been long gone from that area.
68-80, seems to go at a more reasonable pace, however not moving to Northrend at 68 greatly delays the rate at which you could level. It moves to Northrend around 70-75'ish.
Progress to 80 at that point is a bit slow due to it rocking the starter zones (the ones you SHOULD be doing at 68), instead of moving to the zones more appropriate for the level. At 78, you end up doing the zones you should have been doing at 74'ish, and grinding those out until 80.
Now from 80-85, this is where things pick up, but are still a bit slow (from my leveling experience). At 80, you have one profile to load -- Hyjal. IMO, you get much more XP in Vash'ir, and it goes a lot faster. It is too bad there is no profile for that (which I could find), else you could easily grind from 80-83/84 doing Vash'ir alone which only takes a few hours to complete for the achievement and mount. I suspect this is because the bot may have some issues in trying to fight the superman shark or some of the insane elites down there -- dunno. Back on Hyjal, the script bottles out on a Blizzard loot bug and can make no further progress. You are then forced to switch profiles -- which thankfully, you are just barely high enough for it to load the 82-84 profile -- else you would have to grind some. This may vary depending on the amount of BoA's you have, and if you get guild XP bonus. You may still have to grind as the level at this point with a full set of 85 BoA's and level 25 guild bonus ends up just barely into level 82 ---
85-90, i couldn't be bothered to watch the whole time. It does like to aggro a shit-tonne of mobs then stand there and try to fight them off --- this is where being a plate wearing healer would come in handy -- A LOT. Else, I suspect your death rate will increase, and your leveling rate will decrease. 85-90 requires approximately 150-200m xp (don't quote me on exact for this, never bothered to look it up for exact number -- but it's a lot, and leveling is VERY slow). In the time it takes to level 85-90, I was able to level two other toons to 80, and a third to 40 --- THAT is how long it takes.
All in all tho... here is my advice on leveling fast, and rating.
Questing - Fastest
Questing + Dungeons - Bit slower than just Questing -- and this includes doing the in-dungeon quests for more xp.
Dungeons - Slower than both the above. When compared to just questing, the time it takes for you to level in dungeons you could gain 2 levels from questing. So essentially cuts leveling rate to 50%
Grinding - Extremely slow. Even slower when in a party. Don't be fooled, RAF XP = x 3 per kill, but, the XP is cut down to about 25%, so you really end up with less xp than just killing the monster outside the party. If you choose to grind, better to have two bots grind outside rather than in a party -- even if on RAF.
Battlegrounds - This xp is absolute garbage. The ticks make it look like it goes fast, but it is even slower than grinding. Rating for battleground xp on a scale of 1-10... 10 being best .... is SHIT .... less than 1 -- even if combined with some of the BG quests ... which are also shit.
This is from my experience of leveling 107 characters, with/without friends.
End result, even though Kick's scripts are not as efficient at leveling as they could be, from what I have seen so far (and I havn't seen them all so if someone has better, more efficient AFK questing scripts, let me know or pm me.), this is still your best option if choosing to bot. Or, you could use the bot for combat only, and do the quests by hand. Zygor is a good guide that does work well, and does fall in range of switching zones at appropriate times, to the appropriate zone, but costs money unless you endeaver to snag from a site deep in the cellar of the internet.
Hope this helps.
. If looking for fastest, 100% for sure, questing is the way to go, and if doing by hand, do the dungeon quests only in addition (eg, don't queue repeatedly, just queue for the dungeon once, and do the quests inside, then untick it off your todo list). While questing tho, you may find you outlevel the dung quests, so will need to unqueue, and ramp up for the next line.