I am so obsessed with Amano Chigiri's art in Chou no Doku. I love how her style perfectly captures the beautifully dark, moody atmosphere of the game while still being so delicate. As far as I know, this was her only otome game project, and she mostly does illustrations for Josei romance novels.
I also love Satoi, whom others have mentioned, and Teita(Norn9, Nightshade)!—their art style is just so soft, round, and cute.