Here's my pick:
*Clannad - ain't gonna put this in question, simply because it's beauty is WAY beyond measurement.
Top 5:
-Death Note - the feeling of empowerment you get from listening to the choir + orchestra pieces is just intense, and the other songs fit just right with the general athmosphere the anime is giving off. Also, all the theme songs fit their respective characters well.
-Angel Beats - Key.
-Mistudomoe - the theme songs fit the triplets perfectly, the OP is just kawaii~ and the other in-episode songs are nice as well (don't know the names of those, however). Also, ~po ta chi te~ was heavily underused imo...they should really make a ~po ta chi te~ song in the 2nd season.
-Samurai Champloo - I don't know what to really say about this. It's just full of win.
-Haruhi Suzumiya - "God knows" and the OP of Season 2 are my faves from the series. Didn't listen to the whole OST though.
The OP of Arakawa - UtB was nice as well, though I haven't actually listened to any other songs from the OST.