Since there's a lot of composers out there as well as different eras, I'll do this by splitting between the 20th century and the 21st.
If it doesn't go that widely, how about just mentioning any of them you want?
For pre-20th century works, mine would be Kōhei Tanaka, Yasunori Iwasaki, Kōichi Sugiyama, Toshihiko Sahashi, Kō Ōtani, Yasuo Higuchi, Chūmei Watanabe, Takashi Kudō, Yōko Kanno, and Tomoki Hasegawa.
For 21st century works, Yasuharu Takanashi, Takayuki Negishi, Kenji Kawai, Makoto Miyazaki, Kazuo Nobuta, and Taku Iwasaki.
All these people have made OST's that I actually downloaded and listened to for a long time, would it be the same for you?