Tough one, at his peak, it is hard to pick anyone against Ali. Joe Louis is the only one I can think of, although his lack of foot speed vs Ali bothers me. I think he tries to cut the ring off and look for weaknesses like Ali's tendency to get hit with the left hook. If he hurts Ali, his precise hard punches may be enough to get the KO or maybe he loses but Louis seemed to always do better in the rematch - maybe he wins the rematch.
Tyson could catch him leaning back with a left hook that decks him. Ali gets up then get finished with a brutal combo.
A prime Liston imo was the most complete boxer in history. I wouldn't favor anybody to beat, let alone knock out a Prime Ali. But Liston and Louis would've had the best chance imo.