Ian McDiarmid hands down. His performance in ROTS and ROTJ in particular, still rate as one of/ if not the best archetypal evil portrayal of a character I have ever seen. And his ability to play the sly, cunning senator/ chancellor were magnificent as he always had that "I know EXACTLY whats going on but am fooling all of you and I'm so gonna own all of you....in due time" feel to him in EPS I and II.
Next to Ian I'd say Ford, then prolly Ewan McGregor.
Although, as a kinda runner up I'd give best Jedi character portrayal (not including Frank Oz for Yoda) to Liam Neeson as Qui-Gon. I thought he did a spectacular job as Jinn in Eps I.
Harrison Ford. As someone said before he didn't look like he was trying. And he still pulled off a believable performance. Ian did great as the Emperor,so #2, he seems like an old grandfather type person in ep 1 and 2 then he shows his true evil in ROTS. He does it so well. Guinness tied with Ian for 2nd he does his job well, as a mentor. And he tells Luke everything so seriously, it's so believable. He tells Luke the events of the prequels and for people before TPM came out, were really going on what he said. It built up anticipation, for me anyways.