Air TV OST
how could I forget that one!
for the moment i have to say tengen toppa gurren lagann !
the ending for ep 16 for example, or the song during the kittan/yoko scene (and the action following afterwards) in ep 25. the soundtrack is coming out on wednesday.
I'd like to add Mushishi to the good-ost anime list, it's really cool and relaxing (the opening song is simply great).
Paranoia Agent and BECK have also good osts