I never thought of Star Wars as being genuine science fiction, but as far as being a fantasy film I can't think of many better examples of cinematic storytelling (all the films together, that is, not just the first.) Retrofitting or not, the character revelations that occur throughout the trilogy feel entirely organic. The whole Darth Vader/Luke conflict at the end of "The Empire Strikes Back" was satisfying because it came as a surprise...it denied the truth of the universe as we knew it, defied expectations (both ours and the protagonist's), while making complete sense (both logically and emotionally) in retrospect. By the end of the third film you really do feel as though you've experienced a character journey of sorts, which I think is more than can be said about even the most thought-provoking of science fiction films.

I do like 2001, but for entirely different reasons. As pure cinema it's nothing short of gripping...I love the long stretches of silence/music, and the use of minimalist sound effects (for example, the astronauts' breathing) to completely engross/unnerve. I haven't seen Forbidden Planet but it sounds intriguing!