Always been a fan of supercell here, along with a lot of the Lantis singers out of NicoNico, namely Choucho. Maeda Jun composing for Lia or Tada Aoi is always nice, and I'm looking forward to his collaboration album with Yanagi Nagi (nagi). Kalafina is nice for the Kajiura Yuki instrumental. They did a great job with Kara no Kyoukai and Madoka.
Rambling some more: YUI, Kobukuro, Remioromen, Baseball Bear, Galileo Galilei, Falcom jdk Band, sound holic, MARiA/GARNiDELiA, Jam Project, eufonius, mosiac.wav, X Japan, Key Sounds Label, I've, Flow, AKINO, some more I'm forgetting