Miguel Cotto is seen as a legitimate opponent to take away Floyd's 0, and is quite possibly the most dangerous opponent to Floyd aside from Pacquiao. Sergio Martinez is a little out of the book because he's still fighting as a middleweight, but if Mayweather wins next Saturday then talks between the two can start as Floyd will be the king of 154 should he succeed.
The two were supposed to fight years ago when both were undefeated and under the Top Rank banner, but things fell and the two went their separate ways. Two losses later, Cotto is ready to take on Mayweather in the biggest fight of his life. It's as they say, it's always better late than never.
Although this fight is already perceived to be a shut out by Mayweather or an easy win, it'll be a competitive fight. While Cotto isn't in his prime anymore and "shop worn," he's hard hitting, relentless and tough. He'll present a much better challenge than Mayweather's last three opponents.
Ortiz was strong and young, but his inexperience killed him. Marquez was extremely skilled, but he was fighting a style that didn't fit in with him two weight classes higher than he usually fought at. Finally Mosley, while at the time hard hitting, was old and after the second round it became target practice.
Also, Cotto is the third most profitable PPV star today behind Mayweather and Pacquiao. In his last fight, he supposedly sold about 600,000 PPVs with Margartio. With his strong fan base and popularity, that'll be sure to boost the PPV ratings.