Sethekk halls seemed to me to be the best.

I boost from there from level 64, entry level. Dual-specced my 80 pally tank as ret, having about Naxx entry gear level, slightly below. I can clear that place in under 12 minutes, meaning I hit the reset cap. 15% a run at start, 10% a run by the time you're getting close to 70.

You only have to move your guys twice to get all the exp from the place. First they just camp at the instance entry. Secondly they camp in the little alcove, to the right, in the room where the first boss is.

Boosting anywhere is hard, but this seemed to be the easiest to me. Only problem is the mind controllers, though you can usually bring them down fast enough, even with my bad gear, to avoid this. No other main problems, though a non-healing, or cloth class might have trouble with the amount of physical damage they still put out.