50. Wizard/Ur-Priest/Theurge/Inc/Dweomerkeeper/Factotum/Swiftblade...you know the drill, "make dumbest X-level character and try to kill equals/gods"-type game. That said, at that point levels hold little meaning anymore as the game has devolved into a chess match between best uses of epic spellcasting, and finding holes in your opponent's impenetrable defenses with your dozen actions per turn long ago.
It's about the same to play level 30 as it is to play level 100, at least without heavy restrictions to skill boosting/if epic spellcasting were derived off another stat instead and mitigation were heavily altered. 'cause the only restriction at that point is how high a Spellcraft check you can make, and within reason, the spells of practical use are all eminently doable on 21.
Though from scratch? 21. Certain Elf Gish.