My favorite, as in the author who's work has had the biggest effect on me and my life, is probably Marya Hornbacher. I've read both her mental health memoirs (Wasted, an eating disorder memoir, and Madness, which is about her bipolar disorder) about a trillion times each. Madness is my favorite book of all time, even. I have bipolar I like she does, and I first read Madness during a manic episode. It was not only a comfort, but helped me understand my own illness better. Sometimes I would think or feel something, pick up the book, and read those exact thoughts and feelings in the book! And this was before I even knew I had bipolar (I first read it four years ago, when I still had my old diagnosis).
I haven't read her other books yet, but I've bought them and am making my way through them in release date order.
I could also write an entire essay about how Marya's writing style impacted my own writing and inspired me to write better, but I don't want this post to be too long.
There are plenty of other authors I like (such as Suzanne Collins and Lemony Snicket), but Marya had the biggest impact on me for sure.