Geez, this is a tough one. Back in the early 90's, i would have said easily said Ken. As time goes on, i've grown to appreciate the rest of the cast and can make cases for each being the best one. I currently thinks it's Zangief. Hmm......gotta go with Ken. He was my main character the longest.
You've never played it?! It's completely different than other wrestling games, especially PS1 Smackdown. The grappling is timing based, with a hint of rock-paper-scissors. Striking comes in three flavors, weak, medium, and strong. There are tons of match types, including barb wire and landmine matches. I can get into the specifics of the gameplay, but it'll take a while to explain it. It's surprisingly deep and strategic.