I'm a big fan of OST's. Besides giving you the opportunity to relive certain movies, it's a great way to discover new music, to find back the forgotten 45's and in some cases to find an unknown brilliant. One of my favorites is Tarantino, I think everyone enjoyed the Pulp Fiction Soundtrack. But also our dutch one-hit-wonder Little Green bag on Reservoir Dogs and the absolutely rocking Kill Bill I and II. Jackie Brown made me listen to the Delfonics for several weeks.
Other great soundtrackers are the Coen brothers; Bob Dylan's The Man In Me on The Big Lebowski and for example Paul Simon's Boxer on Intolerable Cruelty.
Vanilla Sky made Sigur Ros known internationally, the Girl Interrupted soundtrack was a lot better than the movie, Virgin Suicides launched the french group Air, Yann Tiersen provided a beautiful soundtrack for Amelie, I really enjoyed Gary Jules on Donnie Darko, and Luz Casal was absolutely brilliant on Almodovar's Tacones lejanos.
I'd also like to mention Ennio Morricone, Luis Bacalov, Henri Mancini, Ry Cooder and John Williams.
Let me know if you have a favorite soundtrack.