He still won that fight by submission. GSP has been a dominant champion but fedor face better challenge in pride when he was the champion.
In terms of overall career i think fedor would still be ahead, as for the biggest challenge of their respective careers, I don't think Fedor has faced someone heavier than him with as much talent as Anderson Silva.
Fedor is much more of a natural fighter than GSP is, but GSP makes up for it with his raw athleticism and sharp skill.
Oh tell me more, sir. On average, how much weight difference does GSP or Anderson Silvia normally have to face, is it 10lbs, 20lbs, 30lbs, or 70lbs during weigh in. Sharper skill you say? Fedor excelled in Judo and was an elite Judo fighter in Russia...IN RUSSIA, he is also an elite Sambo fighter as well as being an elite MMA fighter. Fedor is also regarded by the best in the business as having a very good standup (gfrom Holland's best kickboxers). Outside of MMA GSP hasnt really achieved as much as Fedor and clearly does not have as good a skillset.
When GSP goes up, i'm guessing the weight advantage for Silva won't be over 10lbs on fight day. I dont disagree that Fedor fought ridiculously big opponents, which we're still very skilled, but none of the brutes Fedor fought were as complete fighters as Anderson Silva. GSP vs Silva has to be a at least in the top 3 superfights in mma history. although, as I mentionned, in terms of whole career Fedor is pretty much untouchable.
Should it ever happen, I think whoever wins out St. Pierre and Silva would have a very special place in MMA history. It would be the first time Number One and Number Two Pound for Pound have fought and the winner would be the first undisputed P4P King. If it was St. Pierre who did it would be even more special given his victory over Penn and his unbelievable sequence of not dropping a round for over three years. That all said I think Chael Sonnen would be a tougher opponent for St. Pierre than Silva.