Oh man, there's a ton of anime composers i love. I'll name a couple that instantly come to mind.
Tenmon - Really brings out anime like 5 Centimeters per Second to life. You can't help but feel sorrowful, hopeful or (dare i say) tearful when these pieces appear during the movie. Maybe one of the reasons i love his work is because i'm a sucker for classical music. Makoto Shinkai (director of 5 Centi per Sec) really needed Tenmon's talent to draw out the energy and emotion from this movie. Not that the movie wasn't good on it's own.
L'Arc~en~Ciel - My god, i love this group. First heard their music in Fullmetal Alchemist (READY, STEADY, GO!) and it was [tornado fang]ing awesome. Their opening theme for Gundam 00 was phenomenal as well. Something about their fusion of pop and rock just works so well, and is always catchy and exciting. I definitely need to hear more of their music.
Yuki Kajiura - I simply enjoy her mix of synth/pop with orchestral sounds. Also the singing, dat fukken singing.
This. Her composition for the Kara no Kyoukai movies are stunning. Her style is not something you often hear in other anime. Knk is the only anime series i've heard her work from, but i know that she is really talented and creative with her orchestras.