Obviously, I'm not the first to say it, but Mayweather needs to fight the winner of Cotto-Pacquiao, and we'll go from there... If he beats Shane Mosley, people will say, "oh, he beat an older version of Shane", so I don't see why everyone is clamoring to see that one...but, after Floyd fights whoever wins on November 14th, then yes, Mosley would make an ideal opponent.
The only reason I'm tossing Cintron into the list is because he has a punch (or had a punch in 2007 anyway) and Spinks can sometimes be interesting in that he is a pure boxer and his disjointed style sometimes throws people for a loop. I also tacked those guys on because I requested top 5 fights and I don't think outside Williams, Cotto and SSM there is a great deal of depth of people for FM to face. Berto seems unproven, but I guess I'd toss him in there along with Cintron and Spinks. Pac is a great fighter, but he is just too small to make me think that a win over him means all that much. While his victories over Hatton and ODLH look fine on a resume, both men were on the extra rapid decline by the time he faced them. A decisive and conclusive third win over JMM would have been worth more than victories over both men combined. I realize this is not a popular opinion in this world of everyone wanting to cross divisions, but harvesting little fighters does not a great career make. I guess I'd just like to see Mayweather fight someone that could actually do some damage if and when they connected. Floyd is in this solely for the money and to pay off Tyson-like debt, which is fine, but let's not make his current run any more than a WWF-like spinoff on boxing with a few quick payoffs before he rides into the sunset. Until he faces some real Welters or a Junior Middleweight coming down, I'm not sure what the point is in saying any of this is worth anything to his legacy. And don't get the opinion that I'm putting any of Floyd's skills in doubt for a second. Frankly, I think I'd lay odds on him against all of the five men I posted. It's just frustrating to watch him waste this last run on fights that mean absolutely nothing.