I think Mosley has the longest reach among Pacquiao's opponents. This 7 inches reach advantage could pose problems for Pacquiao.
Mosley doesn't fight long though. He's a straight up slugger. Don't think reach will be much of a problem. Body punching and his right hand for me. If Mosley is successful landing many of them, he will end Pac's world.
Mosley will probably try to box Pacquiao at first. The reach will be a factor. He can set up his right hand with it.
I hate to say it, but I don't think Pac will have any problems whatsoever... I said others, which I guess means Pac's biggest issue will be deciding whether or not to carry him to decision. It's still a pretty shameful fight to be made in my opinion, just like with Margarito. For the sake of not sounding like a troll, from the other four choices I'd go with Shane's 7 inch reach advantage... If he can keep Manny at bay and not eat shots early like Cotto and Margo, he could possibly grind him down later. Not if he disappears for ten rounds though.
Pacquiao's biggest problem is the possibility of the SSM of 2000 traveling 11 years into the future and fighting for the SSM of 2011.