IMO it has to be Pavlik. He beats Zertuche, brutally KOing him. He fights Edison Miranda, who almost everyone (including myself) picks to beat Pavlik soundly. Pavlik KO's Miranda after a brutal 7 round fight. Then he gets off the floor to KO JT, the middleweight champ who beat Hopkins twice, drew with Wright (Winky won IMO, but its impressive that JT could even hang with him and get a disputed draw).
There have been some impressive performances, but Pavlik has had 2 huge fights in which he was the underdog and he came through both times, he went from being considered an overrated, protected prospect to middleweight champion.

If Hatton beats Mayweather you could make a very strong argument for him as well, but I doubt that happens.