I think Spinks would have been a tough fight for PBF and would be a closer fight than someone like Manny...IMO.
jmm is zero parts pressure fighter and has shown when he comes out of his defensive shell to attack, he's very vulnerable. (see pac/jmm2) he was not going to outbox floyd his is bigger and much better at all the things jmm does. intelligent pressure is the key. an aggressive counterpuncher will give him the most difficulty. jlc, zab, corley and augustus are the only ones i can think of that had any form of what u can call success against him. cotto will bring something to the table that floyd has not seen in quite some time. cotto is a very intelligent fighter with great timing and ring IQ. he will not be wreckless or lose his cool when something isn't working ala ortiz. he has depth. this is going to be a chess match.
I think SRL and Hearns would both put a bit of a hurting on Floyd, I'd put money on them beating him. Whittacker would be a really close fight, would be amazing to watch but I think Whittacker would just shade it. The Duran from the first Leonard fight would also have a good chance of beating him.
But at 130 Floyd was unbeatable it seems. His speed was other worldly, power was deceptively good, and used his legs a lot more effectively than what he is at welterweight. Do you really think at 130 JMM could be competitive against the version of Floyd that annihilated Corrales? JMM lost to John at 125 and could barely handle a one-handed Pacquiao at 130.
At 126 and 130, I say its a chess match, May has never fought an ATG fighter like Marquez in his prime at 126 or 130, It would be very close, but many people see Floyd as top 20 ATG, untouchable, and Castillo 1 never happened.
Floyd was seemed unbeatable against lesser opposition to a prime Marquez, Pac is another ATG, Pac vs Marquez are two ATG fighting each other, and Marquez more than handled a two handed Pac, it would be much closer than every one thinks.
Napoles, Leonard, Gavilan, Hearns, Whitaker, Duran, Chavez, and many others would probably give May losses.
It will take a special fighter just as he is and is dedicated tot he sport just as he is to beat him....maybe it is Pac...who knows. JMM gets beat at any weight...def more competitive though at around 130 simply because JMM would have better speed of his own and more comfortable but Floyd at 130 is ridiculous nad it would take some serious talent not to say JMM is not good but he is not on Floyd's level and thats not a knock on him either.
Marquez has trouble with fighters who don't attack recklessly. See his matches against John and Barrera. Floyd is one of the best counter punchers in the game. You top that with him being at the prime of his career and it's basically unfair to his competitors. Marquez is my man and I'd love to see him beat Floyd but...I recognize God-given gift when I see it. At 142 JMM appeared slow...but maybe that's because Floyd was so much faster. A cat can appear fast. But put it in a foot race against a cheetah and it'll look like a turtle.
For actual boxing fans who enjoy skills on show it would NOT be boring! For "boxing fans" who just like to see slugfests it may be boring.