I know casamayor fought him and lost a close one but he would've been his toughest. Campbell would've given him a good fight though.
imo, no one. even if 135 looks good now and some fighters getting recognition, i feel that pac and marquez can clean this division by themselves, then add joan guzman to the fray and its all the 130 guys dominating 135. maybe campbell, if he performs another juan diaz type of performance, has quite a fair shot of beating a castillo that fought and arguablly won against floyd.