Perception wise, I don't think it is even close... Fedor all the way... Fedor was pretty much considered un-beatable at the time, while Cain has recently lost badly. In reality, I think beating Cain will be a slightly bigger feat though. Hindsight being 20/20, Fedor was nearing the end of his career and had already started to show some *****s in his armor prior to the Werdum fight.
4. Big Nog