I think his style is so well suited for WW that I can't really imagine anyone currently in the weight class beating him. Obviously Serra already has, but comparing the GSP of then to the one of now would just be a showcase of ignorance. I think Georges work ethic is probably the most well-known in the sport. A lot of people talk about it, and I can't help but think that is one of the major factors of his dominance, his will.
I don't see anyone beating GSP anytime soon, at welter weight. I think a bigger weight class where he'd have to worry about heavier hands would be interesting to see him in, but I'm not so sure we ever will.