Hard question to answer, he has actually had a lot of great fights even though people were soured by the end of his career. I will probably go with the rematch with Hughes when he first won the belt. It was a long time coming, and everyone knew at the end of that night that the WW Goat had arrived.
Runner up is GSP/Penn 2. I am a big GSP fan, and there was a LOOOOOT of shittalk on the boards from Penn fans about Penn being robbed in their first fight. Penn fans were basically the Conor fans of the day, and it was so satisfying to see a fighter I liked trounce Penn and silence his obnoxious fanbase.