Well since Fedor's last big win before Sylvia(Cro Cop August of 05) Fedor only has three good wins Cro Cop, Hunt, and Sylvia. Whereas GSP has 7 good wins Trigg, Sherk, Penn, Hughes X2, Koscheck, and Serra.
Serra isn't even close to a top 10 fighter, and sherk and penn are 155 guys. I wouldn't consider Trigg a top 10 fighter either. With that being said, fedor has beaten great fighters, he simply hasn't been able to get good fights lately. Crocop, sylvia, big nog, all impressive wins. To add more detail, he DESTROYED tim sylvia, the man who made randy take 25, while he had a back injury and is a 2 time HW champ.
As for silva, he is dominant in a thin division, and IMO lost the first round to hendo.
Fedor simply dominates everyone, including the best fighters in the world, and I simply don't see anyone else doing it on his level. He out struck someone with a massive reach advantage, who is billed as a striker, and put away the person who is probably #2 in the world in his class, in under a minute.