I'm a huge fan of (good) anime, and one of the key factors for deeming an anime as "good" is the soundtrack--Not just the opening and ending songs, but especially the background music, and how it enhances the overall feel of the anime as well as the ambience of specific scenes, whether serious or freakin' hilarious :D
For example, I've been listening to THE THIRD ~Aoi Hitomi no Shoujo~ almost non-stop now because of how great the soundtrack is. (It's a pretty damned awesome anime as well, ranked one of the highest IMO, but surprisingly not as well known as other anime.) It has a lot of "scenic" scenes--panoramic views of the beautiful desert landscape, and the music really enhances the mood. And during fight scenes, the music is intense and exciting, gets you really riled up for the fights :D So, stuff like that :)
Also--not an anime, but still worth mentioning--the OST to Final Fantasy VII: Advent Children is up there, but only because the most venerable Nobuo Uematsu composed the songs. \(^^)/ Ain't nothin' like heavy metal + a full orchestra to get your blood pumping. I recommend anything by his band "The Black Mages" if you also liked this OST (^^)b
I've had the OST from Neon Genesis Evangelion for a while, and the music brings about more nostalgia than anything, but is very, very nice. Especially from the 2 OVAs. Komm Susser Todd is still among my favorites for it's... unique composition (it's like... gospel music genre, but talks about committing suicide? lol <3).
Another OST I've really liked and am going to download in the near future is from Full Metal Panic!/Fumoffu/The Second Raid.
There's a lot out there I haven't mentioned, but that just leaves room for you guys ^^ What are your favorites?