I'd personally agree with Jdun, GSP is the best ever, because he is the best fighter ever. He wouldn't be the greatest champion, the most prolific but he is the best WW, in terms of skill and capability, to ever compete, and therefore he is the greatest WW of all time. It's just a matter of interepretation really.
Well GSP just needs to prove that he can stay on the horse. He fell off against Serra, re-evaluated his career, and came back better than ever, which is the mark of a real warrior. Now hes back on this horse, stayed on it once against Fitch and just needs to continue it. A win over a full focused Penn will be great and couldn't come at a better time for GSP. GSP just needs more time and eventually he'll be able to make his claim as the greatest WW of all time.