Okay, the best soundtrack to me is probably all the Rurouni Kenshin albums. Man-o-man, the music is so serene and peaceful and....and...
The opening and ending themes aren't bad either.
A few others I would have to say would be Cowboy Bebop's soundtracks, because the albums really have TERRIFIC music, which really fits the setting and mood of the anime itself. Another one that's really, really terrific is the Hellsing albums. MY GOD! I love the music! The opening theme just constantly get stuck in your head, and the music played in the anime is great as well. One more good one would have to be Revolutionary Girl Utena, because the music is EXCELLENT. The battle music and the opening and ending themes....are very haunting and beautiful. Ah, well, those are just a couple of mine that I really like.