I see a lot of people suggest recent anime, I'll suggest older
I don't know what genre you are looking for (orchestral or not)
(And try to not suggest the previously already mentioned one)
- Ghibli ?
Joe Hisaishi's OST, like Mononoke hime ~image album, laputa, nausicaa, etc. many good OST.
- Evangelion
- Tsubasa Chronicle (
Yuki Kajiura)
- Noir (Yuki Kajiura)
- hack//series (Yuki again)
Some other :
- Akira symphonic suite
- Berserk
- Blood+ (Marc Mancina)
- City hunter
- Cowboy Bebop (
Yoko Kanno) (many albums !)
- Escaflowne (Yoko Kanno)
- Fruit Basket (Ritsuko Okazaki)
- Haibane Renmei
- Hikaru no Go
- Rurouni Kenshin
- St Seiya
- Touch, and Hiatari Ryoko (though, it's maybe a little different than all other anime I listed above)
Artists you are sure to win with :
Yuki Kajiura
Yoko Kanno
Kenji Kawai
Joe Hisaishi
less known :
Kow Otani
Michiru Oshima
This is only few of them.
I don't understand how you can't think of any anime by yourself, just pick some randomly from any "well known anime" series.
Or just watch many anime, and listen to their music, if you find music you like, just look for its OST.