I am confident that from 58-70 instance buddy is for sure the fastest leveling method depending on your queues. I have gotten all but 1 class 60-70 with this technique, 1 day for healers, 2 days for DPS.
If you're in this range, I HIGHLY suggest trying this method out:
At 58 you can get into Hellfire Ramparts and you can get anywhere from 250-300k xp/hr. As you continue, start queueing Blood Furnace then Mana Tombs (these two are 100% afkable) and you can do that all the way to 68 getting about 250-400k xp/hr, and then Utgarde Keep (also afkable) at 68 for about 300-450k xp/hr. If you consider how much time XP it takes at this level to ding, you are getting about 1 level an hour up to 65, and then a couple hours a level up to 68, then probably a good 4 hours per 68-70..
I have literally leveled from 60-70 in a single day through IB with my resto shaman and disc priest. For non healer classes queue times are worse so I just mix the all BC grinding profile and the XP/hr is only a tad worse than my healer insta-queues.
I think any other bracket of levels is best accomplished through questing though. Going into LK xp required to level is significantly increased, but the xp you get from LK instances stays similar to the xp/hr in BC.
EDIT: Also notice when I say AFKable, it's mostly no risk for getting reported but sometimes your group dies in a weird spot or HB bugs out you will need to check on it still, but I have been able to leave it for 5-6 hours for several characters and come back with several more levels. The only issue is that IB will automatically add instances to the queue when you level up, so you gotta make sure to uncheck the other ones as you level. Other instances might be IB afkable, but I found that Blood Furnace and Mana Tombs are the best xp/hr for queue time. Auchenai Crypts (afkable) is good too if you have instant queues as a healer/tank, but for longer queues, it's too short of an instance to be worth eating your queue for.