Hughes, Serra, Penn and Sherk were huge wins back in the day but they were small for GSP and even if they were the best WW at the time it's still debatable.
GSP showed that he's the GOAT WW by beating Koscheck, Alves, Fitch, Shields, Hendricks, Diaz, Condit that were good big grapplers.
He also derailed some very huge hype trains in the beginning with Trigg, Mayhem and Karo.