The first Star Wars Trilogy (Episode IV being my favourite) is probably the best piece of music ever written, imho. Watching this trilogy as a kid is what made me interested in music (and film) in the first place, so this was a no-brainer. These scores never seaze to amaze me.
When it comes to John Williams' other scores, it's very hard to pick just a few. I've always been a huge fan of Home Alone (the best christmas movie of all time, with the best christmas-like score), but The Last Crusade (Indiana Jones III) and E.T. are also breathtakingly beautiful and equal, if not superior, to Home Alone. Let's just say the guy has written quite a few masterpieces in his lifetime.