In terms of the absolute crowning achievement, then I would have to say that beating Foreman was his greatest moment. Nearly all the boxing writers and insiders who had seen both men train thought Ali's time had passed and that Foreman was the future of the division. They were keeping their fingers crossed that Foreman would only knock him out and not kill him. What Ali pulled on that night is nothing short of magic and one of the great moments not only in boxing but sports history as a whole.
I think the Thrilla was exciting but Frazier by that time had less in him than Ali did. He took so much punishment against Foreman and all the other wars he had been in since the first fight against Ali that it is hard to compare him to the guy who beat Muhammad in their first fight. True, Ali had slowed down as well, but he hadn't taken nearly as many punches as Joe had in his career. The only thing that kept Frazier in that fight was his heart and conditioning, and the fact that Ali was never a big puncher.