This is the first time I'm answering a question on here, since the answers are unbelievable to me. No offense to you, but I'm assuming you're not BIG into boxing since you asked this question. There are weight divisions in boxing for a reason. Tyson would have nearly 80 pounds on Floyd. Mike wasn't fat, Mike was a specimen of fine genetics. So, that 80 pounds is mostly muscle and power.
In terms of boxing mastery? Floyd is probably the better of the two given his dominance on a division. But, heavyweight fights aren't fought like that.
There is absolutely no contest. Mayweather would go down probably as 50-100 to 1 underdog. Its just not fair. Mayweather might be able to avoid SOME punches, but all it would take is 1 or 2. These guys Mayweather fights are 140-160 pounds, mostly on the lower end of that. He walks around at about 140 pounds. Mike Tyson was knocking out guys who weighed 250+ in the FIRST ROUND. Could you imagine what that would do to a body like Floyds? A punch from Tyson has the ability to not only be career ending but deadly at that weight. The commission would never allow it. I understand this is a hypothetical question- I'm just saying. The boxing ring is only so big, he can't run far enough or fast enough to punch and not get punched. It would be a massacre.
Lets pretend we're in Mayweather's corner, just for fun. He would need to do what he typically does to win. Although, he would need to score points by throwing punches (which isn't really a problem for him usually). Even when he blocks Tysons punches, the power will have him being tossed around and off balance (they'll hurt). In boxing, you're going to get hit, the key is to get hit in places where its okay, like your arms, shoulders (mayweather is a master at that). But, it doesn't feel to get hit anywhere on your body and with enough power you're going to be thrown off balance. Its just not possible, Mayweather would even tell you that. Why do you think these guys negotiate catch weights so aggressively? They're usually fighting over 5-10 pounds lol!
Mike Tyson and just about every other heavyweight.