DNAngel True Light (piano ver) is among my fav. instrumental song. Other songs are such as S.A Special Days & Gorgeous 4U, Stand Up (Reborn end ost), Blaze (Tsubasa Chronicle), Super Stream (Infinite Stratos), Fast Forward (Nurarihyon no Mago). There's too much. Can't decide.
I personally loved the opening/closing songs to Gundam 00, particularly 'trust you' and 'tomorrow'.
The '14th song' of d.gray-man is also beautiful as is 'Saya's song' from Black Cat and the entire Wolf's Rain soundtrack!!!
I'd have to say my most favorite soundtrack is for Inuyasha, particularly the numbers by the band Do As Infinity. They do a lot of the songs for the Inuyasha ending themes. My favorites are "Rakuen", "Shinjitsu No Uta", and "Fukai Mori". And not just that band, but many of the songs used for the beginning and ending themes of Inuyasha are awesome. Utada Hikaru is good too ^-^ I really like "Sanctuary" from the opening of Kingdom Hearts II. And the song in Kingdom Hearts I that she does (I can't remember the name right now but KH fans should know which I mean haha it's at the end I believe blaugh ) The soundtrack for Bleach and Naruto are cool too 3nodding Also, any soundtrack for The Legend of Zelda is the best, anyone else's argument to that is invalid blaugh