Of course it's GGG, that doesn't really mean it's a fight we will or should see. Mayweather barely has business fighting at 154, fighting against a career middleweight? I just don't see the result of that fight ever giving anyone satisfaction. And if the fight happened at 154? Mayweather would win, but it would be a hollow victory.
Ain't interested in Khan because it's too easy, but I'd watch it because I never miss a Khan fight. Thurman is the most realistic option, and a very tough challenge. GGG is the most credible and the most difficult.
I voted Cotto because I changed the question somewhat... Obviously GGG would be a huge danger but... It would to be one of those who is a genuine possibility, GGG will never get a Mayweather fight. It will never happen. Canelo was virtually shut out, no way I see him turning that around, it wasn't even remotely close. Thurman is a big puncher so would obviously have a punchers chance but I think Mayweather would outbox him. Khan, just no, no, no. Chris Algieri, embarrassing, just no, no, no I think Cotto did OK all things considered and I think he would have learned a lot from the first fight, given another chance I think could make it very close.
On this list, it's Golovkin by an absurd margin. Realistically? Cotto - and the closer to 160lbs it is, the tougher it would be for Floyd. I expect it to be Garcia in September though.
First King Kong and then Gonzilla. He might be able to rope a dope King Kong like he did that fool Pac but Gonzilla will spit that fire and cause a real problem.