Raiders is my single favorite film out of all of them, though Back to the Future, Empire, Die Hard, and Jurassic Park are all up there.
But—Star Wars is interesting in that I feel it's the only one that built on itself somewhat over the course of the trilogy, rather than sort of huffing the fumes of an initial great film to diminishing returns. So I'm picking it for that reason.
(That's not exactly how I feel about LoTR, though I do feel each entry got worse over the series, and I just plain don't care about it as much as some of the others. I do think the fact that it was the only one that was a complete story from the beginning rather than only in response to an initial success helps it here.)