Ooooo thats a toughie!
For acting ability in the films I think I'd go with these lot:
Star Wars:
1. Harrison Ford (Han Solo)
2. Sir Alec Guiness (Obi-Wan/Ben Kenobi)
3. Liam Neeson (Qui-Gon Jinn)
I'm a bit confused about the non-Star Wars ones so I'm going to leave them for the meantime. I can give you my fave at least though:
1. Sigourney Weaver (Ellen Ripley/Alien Quadirilogy).
2. Kurt Russell (McReady/The Thing) - Thanks for the reminder Godjera!!
3. Richard O'Brien (Mr. Hand/Dark City)
I'm assuming we're going on acting ability in Sci-Fi roles?