I think for me it comes between Vader and Don Leo Jonathan.
Vader was great at just mauling people in the ring and proved constantly that no one could do the David/Goliath matches better. His series with Sting was a high point in his career. I also enjoyed his Dome Show match against Inoki in 97 I think.
Don Leo Jonathan just for his agility. The kip ups, rolling splashes and to be honest I've only seen him in the twilight of his career. I'd love to see him in his prime.
Underrated:
Ray Traylor I think is a pretty underrated guy. He takes a lot of unfair shit for his run in 99 and 00. His matches with Vader in WCW were pretty good and he even drug a good match out of the Barbarian at the Rumble.
So who are some of your favorites?