Mitsuda, Uematsu, Motoi Sakuraba (Valkyrie Profile, Star Ocean, Baten Kaitos, others), Kota Hoshino (Armored Core)... some others which I'm too lazy to list.
Koichi Sugiyama - Most famous for the music in the Dragon Quest Series. Also has done many works in the Anime and Live Action scene over the course of the last 50 years. One of the most influential artist in the way of bringing orchestrated music to the gaming scene and is known to have influenced many current composers.
Koji Kondo - Most famous for his works in the Mario and Zelda series. His credentials stretch much farther as both a producer of music and an overseer of new talent.
Yasunori Mitsuda - Most famous for his works on the Chrono and Xeno series. He has branched out to works in other fields and has been seen less and less in the gaming scene, though he does appear from time to time to make a new OST for a game here and there.
Yoko Shimomura - While not perticulalry famous in and of herself, this artist has done the music for many of peoples favorite games. From Street Fighter II to Legend of Mana to Super Mario RPG to Kingdom Hearts. Her resume is just as impressive as any other of her male counterparts. And in all honesty, more impressive than most.
Kenji Yamamoto - While most famous for doing the music for the Metroid series, this is actually one of Nintendos oldest and most reveared composers which has been with them since the early NES days. Lke Koji Kondo, he plays a dual role of writing scores and overseeing new talent.
Jun Ishikawa - Theres some artists out there that only become famous for doing one type of music or for one series. And this artists is one of those. For the most part, this cmposer has stuck pretty consistently to the Kirby series, but that's okay. Because his unique music style fits to that series perfectly. And most importantly, its really good.
Hitoshi Sakimoto & Masaharu Iwata - Its hard to tell where this duo of composers controbutions begins and ends. But what its known is, they quite often work together, and a lot of their works come out quite good. Their most famous works include such soundtracks as Final Fantasy XII, Final Fantasy Tactics and the Ogre Battle series. However, the list does not stop there. Their pairing seems to be a benifitial one, as it has produced many more OSTs following the success of Final Fantasy XII, including projects for such companies as VanillaWare and Konami.