Woohoo!
First vote in by me!
This is surprisingly hard to answer. The main titles for Jaws, SW, and Raiders are all extremely memorable, and worthy answers. E.T. is kinda cheating, by using the SW theme backwards, and Jurassic Park and Superman are good, but not like the others. Same with Luke & Leia and Yoda. The rest I couldn't care less about.
However, the one that I cherish the most is easily the Imperial March. So good it even gets played these days during sporting games.
Barada