The emperor is seen in all 6 movies, Emperor Palpatine was also known as Senator Palpatine, as well as the leader of the Old Republic, which I don't remember what was called
Obi-wan was seen in all of the movies, as a ghost in the last two however. Anakin/Darth Vader was seen in all the movies. Those two droids were seen in all of the movies. Yoda WAS seen in all 6 movies, he was shown in the last one as a ghost
And that's about all for people who's been there for the whole movie.