GSP or Silva
Obviously we are splitting hairs here and no one can take anything away from Fedor. But the fact is he is a heavy weight and any P4P discussion has to address that heavyweights do not have the same skill sets as the smaller weight classes. Fedor has hand speed and aggression like no one else in his class and that's what makes him so dominant. GSP and Silva go out and prove their superiority against men with similar physical tools and beat them with superior technique. You can't say that happens in the heavyweight division with all the gassing and hail mary punching that goes on there.
An aside to GSP's greatness; Jon Fitch is a world class fighter a worthy champ any other year, and GSP is going to school him.