Anything by Yoko Kanno is automatically great. I have never heard a work by her that I didn't enjoy 100%. She can do anything; classical, techno, world music, jazz, rock, you name it, she can compose it. Literally all of her work is excellent. But the best of the best, by far, (IMO), is her work on the Turn-A Gundam series. It is perfection, especially the second OST, Diana and Kihel. Escaflowne, Earth Girl Arjuna, and Macross Plus are also pure win and awesome. There is a unifying element in all of her music though, and the only way I can describe it is as an almost organic "earthiness", if that makes any sense. It permiates nearly all of her work, even the more classical-sounding songs.
I also own many, many other Anime OSTs. Anime music as a whole is generally very high in quality. Some other OSTs I would recommend are the Noir OST (1), Gunslinger Girl OST, and the Meine Liebe OST. Another favorite that I recently discovered is the music from ARIA: The Animation. Choro Club rules!