Okay, assume we make it clear to 20th level. I'd have to go with the Knight. Why? easy answer: nigh-immortality.
The knight's "loyal beyond Death" ability allows him to continue fighting, reguardless of how many hit points he's lost, just as long as he keeps spending challenge points for every round he's on his feet. At 20th level, a knight should have 20 challenges, plus everything from their charisma. That could translate for fighting as if they were still alive for two stright minutes. In my experience, most fights end before 10 rounds are even expended. If it's bad enough for the 20th level knight (with d12 hit dice) to go into the negatives, It's gonna be a "final stand" type encounter, and it's good to be able to finally stand for a good, long time.