Harrison Ford was incredible in Star Wars, he’s a charismatic shot of adrenaline to the movie as soon as he appear in the 2nd act. However his performance gets a bit broader in ESB, and by the time of ROTJ he’s become something of a caricature. Guinness also sleepwalks somewhat in ESB and ROTJ.

Cushing is definitely a good shout but considering he’s only in the first film of the trilogy, it’s hard to award it to him.

I’m tempted to say Anthony Daniels or Peter Mayhew. Both created incredibly iconic characters under very difficult performance circumstances.