I'd have to say No Mercy was the best simply because it was the fourth version of that 'franchise' and had the most robust feature set. But if you tell me Revenge is the best, I can't really argue with you.
It was actually the fifth game in the franchise since WCW Vs. the World on the Playstation was the first. And yeah, No Mercy was awesome! I prefer Revenge though because I was a WCW fan plus it was faster and had a scoring system.