GSP lost to Matt Serra, legitimately. He also lost to Matt Hughes legitimately.
Fedor has NEVER lost to anyone legitimately and has, too, fought the world's best (sans Natural).
I love GSP as much as the next guy...and maybe in a power rankings format (which judges how you are currently), he's be a 1b with Fedor, but if we are talking legitimate P4P greatness, its Fedor and then everyone else and its not even close, really.