Bruce Lee - forget what he did on film, there has been much written about Bruce's fighting skills as a martial artist and not a film star and from what I have read, including many first hand witnesses, I believe Bruce would defeat Ali. His speed would be far too much for Ali to handle and his knowledge of strikes would incapacitate Ali in no time. There's no doubt that Ali would have a puncher's chance but boxer's are notorious for struggling badly in environments which allow attacks to the legs and the use of legs for striking.