The winner is the one who poisons the other’s water bottle before the fight! 
Seriously… either one could clearly render the other essentially helpless by landing one good blow. Whether it’s a KO, a shattered knee… whatever, the fight would be decided by that one blow.
Both are probably savvy enough to know what that blow is, and go for it at every opportunity.
Either one is fast enough to get that blow in before the other can dodge, grab, or block, as far as we know.
Thus, assuming no weapons and no opportunity to ambush each other, the fight will most likely be decided by some combination of ferocity and chance. I’ve seen this happen over and over.
I think I’d give Ali the edge in ferocity. His business was to actually pound on guys until they fell down, while enduring a pounding in return. How often did Lee actually do that?
If chance prevented the landing of the deciding blow early on, I think Ali’s size would work against him in the long run, due to fatigue. It might take a while, but ultimately Lee has less mass to haul around.
Ali would have the advantage if the fight became more of a wrestling match (it probably wouldn’t, though), because of his strength and mass. He could pick Lee up and throw him into things, or hold him in a bearhug or something and pin him down with his weight. Lee could not do the same to Ali.