McDiarmid was excellent in all the films in my opinion. The guy is a classically trained theater actor, you don't get much better than him. Obviously he played "The Devil" himself in the Star Wars movies and The Devil LOVES himself. The Devil is confident, when the Devil isn't confident or feels he needs to tread lightly is when he becomes his most cunning, protecting himself, tempting those around him to turn on one another for his own personal gain. The Devil was at the height of arrogance and confidence in ROTJ, making his cackling and over the top performance all the more appropriate.