When GSP won the belt from Hughes.
GSP obv made a mistake in their first fight, and Hughes never let him forget about it. I'd always liked watching Hughes, but he just came across as a 'bully' after that point. Always putting GSP down and trying to play on his good nature. You could tell GSP was so uncomfortable with it all, I think the highlight being on TUF when they all went out for a meal and Hughes started laughing at GSP about defending an armbar. GSP just sitting there taking it all in, and Matt Serra turning to him and saying something along the lines of,' I hope you beat that guy, he's a dck'. So watching GSP take him out with a beautiful head kick, and GnP, and Hughes not even knowing what he had been hit with in the post fight interview was just as sweet as you can get in MMA.
When he beat Hughes with an armbar in the third fight, that was just the icing on the cake.