Mark Hamill might not be that good in other movies (I've only seen Wing Commander), but he was great in SW. Perfect Luke Skywalker I think.
Anyway you guys shouldn't forget the amazing voice acting of James Earl Jones. He's got the best lines of them all. I think he's got the coolest lines of all the movies I've seen.
So here's my list:
1. Harrison Ford
2. Mark Hamil
3. James Earl Jones
4. Ian McDiarmid
5. Alec Guinnes
6. Natalie Portman
7. Carrie Fisher
8. Liam Neeson
9. Peter Mayhew (if Chewie's voice also is his)
10. Peter Cushing
I know David Prowse's real voice. Doesn't sound a bit like Vader so it's good Vader's voice isn't Prowse's.
And Kris I think everybody agrees with you. He is good, but I don't really think you can see his performance too well with that cool helmet and those black, sexy boots on and he doesn't even speak. Actually in Star Wars every actor is pretty good in their roles mesa thinking.