Marquez is definitely one of the best boxers at that weight. His trouble with Mayweather will be the awkwardness of Floyd.
I think that if Mayweather fights and beats Marquez and Pacquiao he will truly achieve greatness. These are the two opponents I have wanted to see him face because they are, imo, the top fighters he can face.
Either fighter could beat him, but I think Manny has the best shot of the two.