In a straight stand up fight, Nick wins. GSP has a great jab, a superman punch that hits less then half the time and a spinning roundhouse kick that seems to do more to confuse the opponent then do damage. GSP hasnt thrown a damage relevent kick in awhile. Nick puts together combos, flows with his strikes, switches levels and has actual TKO wins in recent times. That said, GSP would simply outwrestle Nick. I think GSP avoided Shields ground like the plague because Shields is a submission wrestler, same with Kos. GSP doesn't seem to fear or have trouble with strict BJJ'ists on the ground and both the Diaz brothers kryptonite are strong wrestlers.
GSP wins this theoretical fight via ground out decision/wrestling smuthering, but if he kept it standing, the fight favors Diaz, imho.