Where do I start? Being a music lover and a writer, I listen to a lot of music have a lot of anime OSTs in my collection.
I have quite a few favorites that I absolutely enjoy, and would like to find more of in like quality.
Fantastic Children : Sadly, this is a very undervalued soundtrack, with an even more underrated anime (which is very, very good). The soundtrack itself is quite enjoyable, with a few tracks that really stick out. (The Children of Bélfort tracks come to mind immediately). Though what really makes this soundtrack great are its opening and ending themes and their respective variations. Voyage and Mizu no Modoromi respectively have to be the most beautiful opening and ending songs I've ever heard. They flow beautifully, and even though you can't understand the lyrics, it's just a joy to listen to. I say go and listen to those OP and ED themes if nothing else. Now.
Haibane Renmei: This is another beautiful soundtrack that deserves high praise. Each track is fluid with its own style, and there are only a few tracks that don't appeal to my taste. (They're very good, those tracks, I just don't care for their style). I especially love the tracks with the vocals, and what really, really made me love the series was "Ethereal Remains." Good heavens, when that scene came on that anime with that track, I was astounded and it made the moment hit you like nothing I had ever experienced. Good heavens, it made it magical to say the least, and I hope most would agree with me, but meh.
Rurouni Kenshin OVA OSTs: Both OSTs composed by Taku Iwasaki, are awesome and really inspire the emotional impact in each scene. The Reflection (second OVA) soundtrack is still good, with pretty tracks, but it pales in comparison to Trust and Betrayal's. And its soundtrack blows me a way with such inspiring tracks like "War of the Last Wolves", "Shades of Revolution", and the "In Memories" tracks. Note to self: Find more of his works.
Beyond the Clouds: The Place Promised in our Early Days: Here's another offer on beautiful music composition. Its violin pieces are particularly moving, and I enjoy the main themes in the tracks greatly. And what with the director loving scenery type scenes, the music helped set the tone in this one.
Blood+ Believe it or not, this soundtrack is composed by Hans Zimmer and Mark Mancina. If that doesn't get you starin', nothing will. I was surprised myself that they managed to get these top-notch composers responsible for so many great soundtracks, and English ones no less -- and here they are in an animé. (I thought I'd never see the day). The tracks are very good, and set the tone for the series well. Also to be noted is another soundtrack full of classical compositions by Beethoven and such for you classic lovers that is played by the character "Hagi' throughout the series on his cello, I believe.
Castle in the Sky : This is my favorite soundtrack by Joe Hisaishi (or Jo..whatever) because of the main Laputa themes that are so nice, but I like his Princess Mononoke OST is great as well, and I like it. The rest of the artists compositions for the Miyazaki films are good, but I feel these two are the best.
Ergo Proxy : Man, those deep choral tracks never fail to send chills down my spine, (New Pulse, Fellow Citizens) and those two tracks make getting the soundtrack worth it in itself. The rest of the tracks, dark themed in nature, (Including the OP and ED themes) are great, too, and though I like some more than others, I hate none of them.
Eureka SeveN: Never was I more surprised that this anime had a good soundtrack, as I never really paid attention to it while watching the animé, and got it merely on a whim to see how good it was, as I highly enjoyed the animé. I don't like the hip-hop tracks, but the rest of the compositions are very good, with the first OST being notably the very best. However, many of the tracks seem like slightly redone versions of the other, though nevertheless enjoyable and pleasing to the ear.
Noein: The best tracks that appeal to me, in my opinion, are the choral vocal tracks. Not really lyrics neccessarily, but just choral. The Noein soundtrack has just that, with its Sangri'la themes that really stick out as unique, with the rest of the tracks following just behind. As a honorable mention, the same author has done the Scrapped Princess soundtrack, which is also worth trying. Still... Noein's better. Choral tracks, all the way!
Full Metal Alchemist: Yes, I know you've probably all heard this one before. But it's still worth mentioning for its variety of different tracks, and great OP and ED themes (Ready, Steady, Go! anyone). I especially like the tracks that follow the "Bratja" melody.
Tales of Phantasia: Yes, it's good. Perhaps a bit more on the standard side of good, but worth listening to nonetheless.
Last Exile: Very pretty orchestral soundtracks that often sound Celtic/Irish/Scottish...whatever in nature, which is something I really like in a soundtrack. Dolce Triad deserves a thumbs up for making this one.
Inuyasha: Yes, yes... I like the soundtracks to Inuyasha, granted they get mighty, mighty repetitive over time, and after the first or second soundtrack really, you've heard them all. Still, the main orchestral themes of the soundtracks are excellent.
Tsubasa Chronicles and Noir Yuki Kaijiura's name seems quite popular nowadays, with soundtracks that I have never found disappointing and hard to listen to; however, these two are those I consider to be the best. Noir's soundtrack is the only thing I like about the anime, with a blend of vocal tracks which Yuki seems very fond of using. (One of the reasons I love her music) while Tsubasa Chronicles (Especially the first soundscape) is just as amazing, with its crux of the soundtrack being "A Song of Storm and Fire". Best. Composition. By. Her. Ever. With other unique soundtracks, like .hack/sign, you can't go wrong with Yuki.
Wolf's Rain: Yoko Kanno sure has a talent for lyrical pieces (And by that I mean real songs with lyrics that actually mean something, rather than being a choral based). Wolf's Rain has to be her best though, with many enjoyable tracks of this nature, like "Stray" and "Tell me What the Rain Knows."
Final Fantasy: Advent Children: While it may not be animé in some people's eyes. The soundtrack is awesome, often taking directly from the game, not to mention it has the beloved "One Winged Angel" song.
Cowboy Bebop and Samurai Champloo: I will probably be one of the first people to say that I hate these soundtracks and can't really stand listening to them (save for a singular track or two). I.E Mystline. However, I know quality when I see it,..er hear it and even though I don't like it myself. (Jazz, BLues, n' Hiphop, ain't my taste) I know it's very good music that I, as a person, just don't care for, but many do. The sheer quality and uniqueness of each soundtrack is also worth mentioning. Try it for yourself and see.
Whoo. That's it for now, I think.