Just listing some good ones off the top of my head that I highly recommend:
Otogizoushi
Since the anime takes place in both modern day and historical Japan there is a mix of both traditional japanese music and a sort of synthpop sound as can be heard from this song. The music was composer of this soundtrack also composed Death Note's soundtrack.
Togainu no Chi
No album in particular, just anything Togainu no Chi. I've never even read this visual novel/watched the anime but the music associated with this franchise is killer, and as the title says, epic. I can hardly describe the type of music. I guess it could be described as your typical awesome song from an anime, but that goes for every song. I was so surprised by the music, I HIGHLY recommend this soundtrack.
Berserk
Okay, just anything by Susumu Hirasawa, but the tracks from Berserk (this specific album) are so few, but are some of the most memorable songs from Susumu Hirasawa. Not only that, but these songs just don't get old.
Chikyuu Shoujo Arjuna
Again, anything by this composer, who is Yoko Kanno (for your information). I love anything by Yoko Kanno, but I feel the sound track for Chikyuu Shoujo Arjuna is one of her best. Every track sends me into an incredible fantasy world and is inspiring beyond words.
Death Note
Another soundtrack with Taniuchi Hideki. This one is much different from Otogizoushi, but still lovely. Perhaps I just like this for the nostalgia (probably goes for most of these soundtracks, but honestly the other soundtracks are killer), but I mean, it's Death Note. What's not to love?
Shiki
Haven't actually given this anime/manga I try (an attempt, but it bored me, I'm sorry, I know it has a lot of fans...'orz) but this soundtrack is killer. I am a fan of Takanashi Yasuharu, especially in all those Jigoku Shoujo soundtracks, but most of the Jigoku Shoujo soundtracks have the boring songs or just unfitting songs. Everything in the Shiki soundtrack is superb.
Requiem for the Phantom
Another anime I just couldn't get into. This soundtrack is a masterpiece to my ears, but it's just disappointing I did not feel the same way about the anime (it's the animation and colours, I can't stand to watch anime like this one). Forget the anime, you can make up your own world with this music. The soundtrack is mainly with faint, quiet, thrilling sorts of songs, but they are so wonderfully atmospheric.
Noir
I also recommend Madlax and Tsubasa, but this song is just too great to skip over. Anything by Yuki Kajiura is great. A few songs here and there aren't so great, but out of any of the listed albums you will find a few gems like this song.
Michiko e Hatchin
I am just in love with this anime! XD The soundtrack is very unique for an anime. It is a gangster brazilian styled soundtrack that is candy for the ears. Each song has a completely different feel that can make you feel tense, excited, joyous, sad, etc.
Mushishi
This one I would like to present more as a 'help me' case. The soundtrack is beautiful...but I also feel like there are probably lots of soundtracks like this, yet I can not find any soundtrack that is as powerful with such gorgeous tracks as this one. Where can I find more Mushishi-esque soundtracks gaiz, help me! ;3; This is great if you like traditional Japanese sounding music.
I've heard a ton of other great soundtracks, but these are just some off the top of my head..or just looking at my library...XD
If you liked these composers you should check out their other soundtracks. Some others that I recommend from the above composers:
Yoko Kanno:
Basically everything...buuut these are also pretty good
Cowboy Bebop
Ghost in the Shell (a few boring tracks, but also some of her the songs I've ever heard)
Wolf's Rain
Vision of Escaflowne (a few boring tracks, but a few gems)
Macross Plus (another soundtrack as good as Chikyuu Shoujo Arjuna. The tracks feature lots of experimentation)
Turn A Gundam (just one song in particular I love)
Yuki Kajiura:
Tsubasa
Madlax
Le Portrait de Petit Cossette
.hack//SIGN
Takanashi Yasuharu:
All Jigoku Shoujo Soundtracks (a few classic songs are great, the tense songs are dull, though)
Susumu Hirasawa:
All of his other albums. What you think I am exaggerating? I am 100% serious. Listen to everything by this man, he is a genius. There was a time when I got bored with his music (listened to it 24/7 for a few months can ruin the magic), but I have returned to him and am continuously hooked onto whatever new song I can find.
Okie dokie! That's all folks! I may do another one of these...'cause I have lot of soundtracks that I adore~!
