I would agree, while he is not my favourite looking Sith Lord he looks very cool in that scene. I believe this is the scene you are looking for.
This. The scene where he marches on the Jedi temple with a clone army right behind him was what made episode 3.
Hayden Christensen may not be the best actor in the world, but he looks cool as Darth Vader.
(Although personally I prefer this scene.)