Everyone is quick to mention Yoko Kanno in queries like this (she did the Cowboy Bebob and Escaflowne soundtracks - I do sorta like the escaflowne ones), but I'm not really a big fan of hers, so I'm not listing any of her stuff Under your criteria for [mostly] orchestral soundtracks:
I think you might really like Neon Genesis Evangelion's movie soundtrack, End of Evangelion
. The Refrain
soundtrack from the same series is not orchestral, but it's got an awesome remix of the Cruel Angel's Thesis theme song that I can't get enough of.
I really love The Twelve Kingdoms
soundtracks, but part of it may be that I am blinded by my love for the show. Beautiful, simple compositions with one very gorgeous vocal for the ending theme song.
Other series with orchestral or orcestral-ish soundtracks are Crest of the Stars
, Banner of the Stars
, and the Record of Lodoss War OAV
(actually I'm not sure if the Lodoss wars ones actually counts as orchestral). You might like the .hack//sign soundtracks
(most people do), give them a shot.In general, some of the other anime soundtracks I really enjoy:The Revolutionary Girl Utena
soundtracks. Volume 5 is easily the best of the bunch. I believe that one is the soundtrack for a musical version of the anime. These soundtracks may be hard to track down legit copies of. The style of the others is hard to describe and is not for everyone, though.Hana Yori Dango Movie Soundtrack
: The first half is a bunch of jazzy-type compositions I really like. Again, this could be really hard to find.Vampire Princess Miyu
OAV and TV soundtracks are both good, and they have a dark, eerie style that fits the show so well.
A quick mention of other soundtracks I like: Please Save My Earth 1, Mermaid's Scar, Mermaid Forest, Maison Ikkoku, Marmalade Boy Best Collection, Chobits Soundtrack 2, Cardcaptor Sakura.
Edit: Added in a bunch of stuff I forgot