Wizards.
So they may be puny, but wizards are unique amongst the classes. They can certainly dish out the damage, and the high level save-or-die type spells (and Will-save based spells at that) eclipse the martial characters. They don't just stop at artillerists (unlike their brutish sorcerous cousins) but have the whole host of utility spells. Need an item identified? Wizard. Need to get somewhere fast? Wizard. Rogue can't pick the lock? Wizard. The high-level defenses make them nigh immortal (Clone, Hide Life, Contingency, Chain Contingency etc.) and the high-level offense is just disturbing (fancy being on the receiving end of Time Stop followed by five Meteor Swarms?)
Nothing else comes close to their unique combination of versatility, power and survivability.