So many idiots on this thread: On his best day it's Ali period. The reason? He could adapt better than ANY of the above. This is one of those comparisons where you have to look at individual matchups and "likely win" or "win share" type metrics. On a given day most of these fighters could beat the other, but who wins most often, at their best, against all these styles? Ali from 1966-1967.
Regarding the Klitschko brothers: These two are just not athletic enough to handle Ali. Some of you referenced the rope a dope, Ali 1966-7, had yet to discover his rope a dope, and wouldn't have needed it against them, he would have moved so much, so quickly, hitting them from so many angels, both would have been stopped on cuts. Throw in he had one of the best chins in heavyweight history.