I'd say that my favorite classes are:
1. warlock (LOVE mobility, and by 8th level with Fell Flight & Flee the Scene, the warlock has it in spades. Include Eldritch Spear and you can spend all day hovering above your ground-bound enemies, picking 'em off while you're at least two weapon increments from their bows...)
2. druid
3. beguiler (I haven't had a chance to play one much, tho had one as a cohort for a short time. It impressed me enough that I statted up a doppleganger monk/beguiler to use as a villain, pretty impressive on paper tho actual play would be needed to fully assess).
I agree with others who have stated that the most powerful is more player dependent than class dependent.