The guy who really did a good job was Maidana in the first fight. Floyd's hardwired to know what's coming and Maidana did a lot of tricky shit to throw Mayweather off. If you watch those fights close (especially the first one) he would use feints and double up on certain punches and get Floyd to roll and then attack when Floyd was stationary. He'd throw a right, Floyd would roll anticipating a left hook, and Maidana would double up the right and then throw the left when Floyd had squared himself up.

Again though it's very easy to say what might work, it's another thing to go out and do it. Floyd has the best defence in the history of the sport, and with his speed and accuracy he makes guys second guess themselves before punching.