I don't have a single favourite, more like a cluster at the top. Morishima Akiko, Takemiya Jin, Rokuroichi, Personal Colour, GP-Kids, Millet Soup, Hakka-Ya, Aki Eda and Pikachi are the names that come to mind most readily.
Top 3 would've been more manageable, but I understand why you said 1.
I'd go with Sakuraba Yuuki (Personal Color). S/he does excellent SoL, excellent action, excellent romance, and excellent nsfw. If all series were written by her/him, I think I'd be guaranteed high quality yuri in a variety of genres.
Otherwise, I would go with the criminally inactive Natsuneko, for much the same reason.