Act III Keep Levels gives you the best XP/hour ratio (if you can farm it of course). Keep Levels have a lot of mobs confined in space, so you don't have to wander or chase them along a big map (wasting a little time). I used it when leveling in Hell difficulty to raise a couple of levels.
Took me two sessions, about three hours, to get to Paragon level 1. But I was recording info for Cyeron's magic find research, which meant resetting NV every time I got a stack, and alt-tabbing every elite kill to record details.