Hard to say since 3.0/3.5 are a little different from Pathfinder and I'm much more familiar with Pathfinder than 3.0/3.5. That being said, the godawful things you can do with a 20th Level Caster, are well godawful. Things like Time Stop, Wish/Miracle, Power Words, and Summon Monsters sounds pretty good.
If it was Pathfinder- an Exploiter Wizard w/ the Foresight School would be awesome. In a fight like that, I think going first would make a huge difference. With Time Stop and a Rod of Maximize, you could cast 5 spells before your opponent even got to go.
I played in one of these years ago. 20th level, one on one, fight to the death. You got three rounds of preparation before combat began. The caster vs caster fights were terrifying. Lots of save or die stuff, lots of no save stuff. The martial fights were kind of boring because of all of the face to face full-attacking. Anyhow, the guy who won the tournament actually played a monk. On the first round of combat he won initiative, teleported next to the spellcaster, and grappled him. Next round he pinned, covered the caster's mouth, and then beat the crap out of him until dead.