I am personally a big fan of Williams scores, I think he has made more memorable themes than any other (Jaws, Jurassic Park, Star Wars, Indiana Jones, Schindlers List, Harry Potter, etc...) most of those movies could be named by music alone which is good in my book.
Other artists I like are Hans Zimmer, though he tends to recycle music (all the composers do this to some extent, he just does it more). You can really here this if you listen to some of his older movies or "the Rock" and "Broken Arrow" together. I really enjoyed the Black Hawk Down score, I thought Zimmer did an especially great job with that. Other notable composers that I enjoy include (sorry about bad spelling) Michael Kamen, James Horner, Jerrry Goldsmith, and Trevor Rabin.
I guess if I had to pick an all-time favorite soundtrack I would cheat and say Star Wars. At this point I have ~15 CD's of SW music and I love the development of the themes for the various characters over the 5 (soon 6) films. I look forward to hearing the score for Ep 3. My other top scores are Gladiator, Schindlers List, Catch me if you Can, Armageddon (don't ask me, it just seems really cool), and Black Hawk Down.
-spike