Any member of the Shield
Bret: The Yokozuna win at WMX. Really cemented him as the guy who they go to when they don't have a guy. I mean that in the nicest way.
Shawn: In his initial run, Royal Rumble 96, where he dominated the match and won a foregone conclusion. By that point everyone knew where he was ending up and there was nothing to do that would stop him. In his second run, it took until shortly after the WM20 three-way, I'd say, at which point we knew he would sticking around for good.
Undertaker: I'd say this one was FAST. He was doing main events against Warrior before he even turned babyface for the first time, and people were more than happy to buy him in that role. I'd say Survivor Series 91 at the outside, but really he was almost there from the moment he debuted.
Sting: The Flair match. Made him a star for life, no fuss no muss.
Shield: Dunno if any of them are what I'd call bulletproof main eventers yet.
Flair: Certainly by the time of the 82 feud with Race he was there.