Mayweather wins them all on points. In order of difficulty: 1. Pacquiao 2. Khan 3. Bradley 4. Madiana
Agree with the verdict, although Bradley surely is the tougher fight compared to Khan. 1. Pac 2. Bradley 3. Khan 4. Maidana in order of toughness for Floyd. He beats them all on points.
I think Khan's style is a bit harder for Floyd than Bradley's although Khan is obviously the one most likely to get stopped. I also think due to Bradley's lack of power, a confident Khan (say the one from the Judah, Paulie and Kotelynik fights) could beat Timmy. Maidana has no chance whatsoever- he wouldn't even come remotely close to winning a round against Mayweather.
against Khan > TKO in the later rounds (actual, non suckerpunch, legit stoppage) against Pacman > UD 8/4 against Bradley > UD 9/3-10/2 against Maidana> UD 12-0