Stone Cold Steve Austin. It might be the boring answer, but it's the honest one. His combination of intensity, swagger, charisma, promo skills, technical ability, and storytelling is still unmatched.
Kane. Unlike others who were heavily pushed, Kane had the talent and crowd support but never received the same push. Despite this, he managed to stay somewhat relevant. With the kind of storylines he was given and the amount of jobbing he had to do, most top stars would have become irrelevant.