From easiest to most difficult, in my opinion it goes in this order,

1. De La Hoya, if he didn't knock him out inside 4 rounds he'd tire and get UD'd or even stopped, the fight would have to be at a catchweight though.
2. Mayweather, styles make fights in this one, Hatton would out work, out muscle and out land over 12 rounds.
3. Cotto. Very strong and could stand and trade with Hatton for 12, accurate puncher, good body puncher and decent defence, but possibly has a suspect chin which would help Hatton, he knows how to pounce when he hurts a guy.
4. Margarito. Punches just bounce off of him, throws whirling shots from all angles, could land a big shot at any time, but could turn out to be the easiest to fight, if he could box him like he did Ben Tackie.