Floyd from the days when he fought Corrales and Castillo, would wipe the floor with Pac. He's too careful now and, and probably psychologically terrified of losing a match, so that would affect his performance. Currently though, I'd say it's a toss up though, with Manny having a slight edge because of how badly he beats his opponents.
I think Paccy is an all time great but I'm not as impressed as everyone is with his fights int he past few years. He's an amazing fighter who's been fighting guys who've either been withered up, dehydrated, past their prime, or fighting without loaded gloves for the first time.
Clottey was the only recent opponent who didn't have one of those disadvantages, but Clottey's style is so defensive that he was guaranteed to lose to an aggressive, active, fast, and skilled fighter as Pacquiao. Despite that, Clottey inflicted a lot more damage to Pac's face than the other way around.
Clottey is bigger than Mosley, but Mosley is faster, more active, and probably hits almost as hard, so he has that going for him. Mosley's performance against Margarito was also more impressive than Manny's, but that doesn't prove anything. Manny is super fast, and super active, so I think he has the advantage. Plus Manny's coaching is 1000x better than Mosley's.
Manny is the favorite, but I'd put the odds closer to 2:1, rather than the 6:1 that Vegas bookies had it at last time I checked.