Honestly Kotoko is more associated with VN songs than anime. Mami Kawada did the majority of Shana songs. One OK Rock is definitely one of the better j-rock bands, but I'm not really into rock so :x Anyway, I listen to less jpop and more anison but I suppose there's enough overlap. I tend to only pick up songs I like from most artists, but there are a few I actually follow.
Nana Mizuki - easily best singer in anison. my personal favorite singer ever. of all time. forever.
Nagi Yanagi - I think half the time people say they like Supercell, they just mean Nagi. Her collab with Jun Maeda was really great.
Kalafina - honestly don't like what they've put out recently but they've done some really good stuff in the past, most notably with KnK.
ELISA - best at pretty sounding music, but her vocal technique has improved over the years. not incredibly amazing, but I like her music.
I suppose Chihara Minori, Tamura Yukari, and Yui Horie deserve to be mentioned considering how popular they are, but I don't actually follow them very closely.
"It was then that I found the darkness in my heart, but I wanted to become a strong person who didn't yield to such weakness." - Nana Mizuki