[F. Gottwald]

imo, the most powerful is also the most fun: the sorcerer. it gives you a prodigious amount of magic to throw around, and you can really personalise your character depending on the way you want to play: a spellslinging killing machine like irenicus himself? a master summoner who would put edwin to shame any time? a walking fortress, protected with so many different spells to be practically invincible?
decisions, decisions, decisions... far more interesting choices than you have with other classes.
ranger/cleric is a combination with abundant different choices as well, but imo the fact that you are pretty damn good at melee takes some of the gameplay away... you usually end up casting iron skins and beating everything into a bloody pulp, perhaps throwing the odd insect plage at enemy wizards but that´s about it.

but then, try things out, imagine what you would like to do... personally i´m in favour of spellslinging because it´s more involving than most other styles of playing.