Depends. He had a stupendous run in PRIDE, as many have said defeating Rampage and Arona was huge.
In the UFC and possibly overall it was beating Machida. Machida was the unsolvable puzzle, and Shogun made him look human in the first fight, and his icing of Machida in the second fight was shocking. It pains me to say that as a big Machida fan, but it's true. These boards blew up.