Wizard.
Wizard just has so much flexibility.
Do I want to be a tank that can't die? Abjuration school.
Do I want to be the ultimate support? Divination school.
Do I want to be a summoner? Necromancy school.
Do I want to make things explode? Evocation school.
Any problem encountered in the game can be answered with "I've got a spell for that" - Plus some unique spells to boot.
Honestly, my only qualm with Wizard is just how underutilized the Int skills are compared to the plethora of Chr and Wis checks.
