On their best day their is no one better p4p than GSP. His wresling, striking and BJJ is on another level higher then most people in the world right now, not just MMA. He shows better technique and dominance than anybody when he's on, including Fedor and Anderson.
Thiago Alves is a dangerous fighter, but at the end of the day I still don't see him beating GSP. Not even close.
GSP is gonna out-work Alves and use his well rounded skills to transition from ground to strikes and eventually finish Alves with anything he wants.
GSP = p4p best without argument.