Ali was a completely different fighter after his exile. He looked pretty fit against Frazier in the FOTC and wasn't as far off as most people say he was. I don't think Ali was at his prime during his first title reign. The 1960's version of Ali was faster on his feet and as a puncher, but hit nowhere near as hard as the post exile 1970's Ali. None of us will ever know how good Ali could have been from 1967 - 1969. I still think Frazier would have beaten him though, if the FOTC happened during these years. Futch and Durham both said Frazier was ready for Ali in 1968 and 1969. That would have been the real FOTC.
Ali might not as hit AS hard, but he could hit more often, in more fluid combinations, and he could still slice the **** out of guys with the occasional twisting punches he threw.