I would have to put Tyson in my list, just because I always really enjoyed watching his fights. I recently watched several of fights on espn classic. I had forgotten how good he was when he was young. He was incredibly fast, this was probably a bigger asset than his pure strength, which was tremendous. He was always much shorter than his opponents, with a much shorter reach, but it never mattered (until his trainer died of course and his discipline and training went down the tubes with his personal life.), he was a bull, each fist was one of the horns.
So, love fest for a sociopathic, ear-biting, sex offender over, here's my list (in no real order):
Tyson
Ali
Sugar Ray Robinson
Sugar Ray Leonard
Julio Cesar Chavez
I'm going to throw in a sixth, Jake LaMotta, as an honorary, after all I've only seen about two of his fights on espnclassics, but because ya gotta love Raging Bull with Deniro as LaMotta.