Mayweather is obviously the greatest in this era.

Most people agree Mayweather beats Pacquiao.

Notable wins over: Cotto, Castillo, De La Hoya, Hatton, Corrales, Hernandez, Judah, Gatti, Mosley, Marquez, Ortiz, Mitchell
It's clear that Morales, Barrera, Hamed, Kelley would also be in this list if they ever fought. Tszyu also. So you can imagine them all on his record.

I also believe Mayweather would beat Tommy Hearns. Tommy Hearns would do well in the first 4 rounds but after that, he'd get exposed.
Mayweather has the best defense that Tommy Hearns would have ever faced, and thus the onslaught from rounds 1-4 wouldn't be as great.
Mayweather would coast to a 7-5 UD win.

Mayweather Jr vs Duran....that's a difficult one (Duran at lightweight vs Mayweather at his best weight - but not considering weights in a real fight).
Duran is a pressure fighter, and let's be honest...Mayweather did not do too well against Cotto - although he decided to take a different approach and move forward.
Duran can be very sloppy, and Mayweather WOULD take advantage of that.

Mayweather vs Leonard - another hard one. Too close to call. Perhaps I'm leaning slightly more towards Leonard.

I'm being realistic here. Think we're talking about this in 20 years time so by then you would put your hot headed hate for Mayweather aside.
It's Mayweather, Leonard, Duran and that's it. We wont include Hagler because he's a full fledged 160 lber.