It's a tricky question because a lot of soundtracks are simply a collection of songs that had already existed, so I can't really count those. For example, there's a lot of great music on the Forrest Gump soundtrack, but it's mostly music that had already existed (and was not composed for the film itself).
So since I only include original scores, I submit the following list (you'll note John Williams had a hand in several of these):
- Star Wars Episode 4
- Star Wars Episode 5 ("Imperial March" could be the single most widely played and recognized instrumental film score of all time)
- Raiders of the Lost Ark (Not only the main "Indiana Jones" theme, but the music played in scenes with the Ark is fantastic)
- Jaws
- Braveheart
- The Wizard of Oz (not exactly my taste in music, but ya gotta recognize greatness where it exists)
and yes...
- Titanic (I don't really like the Celine Dion song, but the rest of the music from the film is very, very good)