A few weeks ago I would have said Death Note, no contest, but having just finished .hack//SIGN, I'd have to say that one is just as good, if not better. If you guys want to post links (preferably streaming, not download, since that's technically illegal) to your favorites, do it here.
Favorites: - Elfen Lied OST - Claymore OST - Naruto OST (even if the show isn't too good, the ost pwns) - Death Note OST II - Ikki Tousen OST
The Death Note OST's. They're very calming, in my opinion. DMC's is awesome, some songs on it get me hyped up. XD
Cowboy Bebop OST FTMFW. The best song, however, is The Real Folk Blues from The Seat Belts ft. Yoko Kanno. This song is the shiz and a half. It's not surprising, since it's a music-oriented anime. (Voted official 3rd best anime in the world, btw) The Real Folk Blues
;_; ... What a horrible question to ask. How can I possibly answer that. ;_; There are just too many awesome OST's out there that I don't know of. ;_; But I'm lazy and don't want to repeat the one's already said so I'm agreeing with RvR, Spike, and Jade.