Originally Posted by Nintendork
Obi-Wan Kenobi, Anakin Skywalker, R2D2 and C-3PO are in every movie. However, Anakin kind of depends on what you consider Anakin Skywalker and what you consider Darth Vader (I personally always consider him Anakin Skywalker, but when he's in Vader's outfit, I also consider him Darth Vader and refer to him as such, which in this case would be in every movie)- if you consider Anakin Skywalker just Vader without his mask on, he's in episodes I, II, III, V, and VI. However, if you consider him as Anakin Skywalker as just him being on the light side, that would be I, II, III, and VI. But to me, if he's the guy who is Anakin in I and still controls the same person (himself) in IV, he's in all the Star Wars movies.
It's kind of nitpicking really. I mean what do you consider to be Obi-Wan? His physical body dies in Episode IV after all. Likewise Threepio's mind is wiped in Episode III, so one might as well argue he's a "different person" in IV-VI than in I-III. Also Anakin, the good man "returns" even when he's inside the suit.
Plus he's Darth Vader before he gets inside it. If you want to get all philosophical, you could say that "Darth Vader" appears in III-VI, and Anakin appears in I-III and VI. But I think it's still more proper to say that Anakin himself is in all the movies, even if he turns evil for awhile.