St-Pierre. He's just too intelligent (fight IQ), too well-prepared, trains ridiculously hard and with the very best in the world, is always in shape, and always seems to have an answer since he became the 2x WW champ (obliterating Serra in their rematch and exacting sweet revenge). That Condit head kick was perfectly timed and caught GSP out of position as he was hopping back. All that it did, however, was catch him off balance. If you doubt this fact then watch the fight again, at that moment, or the GIF and see for yourself. GSP wasn't rocked, he had his eyes on Condit the entire time he was falling (again, caught off balance) and was able to defend the follow-up finish attempt immediately and successfully. GSP has had just that one close call since he lost to Serra way back in April of 2007.
As was already pointed out, GSP's strength of schedule also trumps the others and give or take one or two guys over the years he's been fighting the absolute best in his division.