There's only 1 fighter I can see as unbeatable and that is a motivated Lennox Lewis. He had everything: size, power, speed, combinations. He wouldn't have let Ali sucker him into punching himself out while Ali hung on the ropes; he would have just given him a 1-2 until Ali realized he was falling behind and Lennox wasn't going anywhere. Ali would have had to come out and box and that flash jab would have been good until Lennox caught him with his own. I hate to say it because I love Tyson, but prime for prime, Lewis wins. Mike had too many psychological issues to beat Lewis. He would have caved in the middle rounds once he realized Lennox wasn't about to be intimidated. That's why he just stood there like a zombie until Lennox put him down (although it took a lot of hard shots).