I am going for British Bulldog although it may be a bit biased, But Bulldog achieved alot throughout his career. And is one of them when you think "how did he not win the World championship?!".
He was more than capable of winning it, but he just never got his shot. Bulldog has had so many great matches over the years. And is a hero for British wrestling fans, and also became an International star in Japan, USA and Canada.
So my vote goes for British Bulldog.