Nog, like a lot of guy's at the time, flourished because of the fact that most people's ground games were not that good at the time. Werdum is a better version of Nog. His ground game is trickier (Nog is just really good at the basics), and his striking is better by leaps and bounds.
Cro Cop was only good in a ring. Stipe and Cain would wreck Cro Cop in the cage imo.
Fedor is a mystery, because he fought so many cans, and it was a different era, but I think Prime Fedor would still make a good run in the UFC.
No they would not be top 3, but Fedor would still have a shot at #1.