Depends on what you mean by best. Most skilled ? Most dominant ? Most important ? If we consider all of these, as well as the opponents and the pre-fight expectations, I would say Liston 1. With the rumble being a close second.
Not his best opponent and Williams was past it but still his performance against Cleveland Williams is something special.
I find it hard to pick out one fight where he gave his "best" performance, but I think taking the title from liston was impressive, and then coming back to the sport and take on Foreman the way he did was unbelievable and something you've got to respect even if you dislike his loud mouth. He really shines in those defining moments can't say that about todays hopeful heavies, they just try to look good and maybe last the distance against either klitschko brother.
The Rumble in the Jungle.Unbelievable performance even to this day.Everyone seems to forget how big a favorite Foreman was in that fight.