Shame about Norris: the man can't even form complete sentences anymore. But, I think Norris would have given a ton of trouble, as Norris was a gifted athlete.
I don't think Duran had the hand-speed to compete with F.M.
I think Ray Robinson would have beaten him, but it would be a real technical fight.
I think there's a high probability that he gets caught by Leonard or Hearns. Both were great finishers. I really don't know. Would be interesting to write a program that takes all of these factors into consideration and spits out probable results.
As much as I admired Tszyu, I don't think he would have much of a shot. However, as a previous poster mentioned, Tszyu was deadly accurate with his punches. If Kostya would have landed the same right that Mosley landed in round 2, I think Tszyu would have had a much better chance at finishing Mayweather.
Whitaker would have proven to be a really intriguing match-up, but I think Mayweather has better defensive skills. Power goes to Whitaker, though. Whitaker was also a very adaptable fighter. I'm really not sure about this one.
I think Arguello would have been another intriguing match-up. I've effectively thrown his losses to Pryor to the wind, as Pryor had that piece of filth Panama Lewis in his corner - a man who has ended the career of many a promising fighter with his illegal tactics. Just look at what happened to Collins.