I find it hard to pick anybody against Mayweather, he's a douche but he's an amazing fighter. But Pacquaio presents different challenges for Mayweather.

He's at least as fast and you've got to be fast against Floyd, but probably more importantly he's got the footwork and footspeed to cause Floyd problems. He doesn't have to his feet set to punch, which is such an underrated thing when going up against guys like Mayweather and Hopkins, and he's got the footspeed to get in and out against Mayweather. In contrast, JMM's footwork and footspeed caused Floyd NO problems. This is very important and nobody ever talks about it. Hatton's footspeed allowed him to get inside on Floyd but Floyd beat him up because he was stronger and a better in fighter. Manny can get in but he won't languish there, he'll get in and get out.

Finally, and this is disregarding Manny's power which you shouldn't do, he's a southpaw and he throws punches from odd angles at times, which is much tougher than a righty because Floyd's defense is set up for right handers, he keeps the right hand up to block left hooks and rolls his shoulder to block right hands, this is thrown off by southpaws and the Mayweather camp has always freely admitted they didn't love him fighting southpaws.

Manny's got a much better chance to win. He also, because he'll FORCE Floyd to open up more, has a better chance of getting KO'd than JMM did. I still haven't made my pick. I've never given serious consideration to Floyd losing so that's something.