I don't usually read sci-fi. I wouldn't really consider Lovecraftian horror or dystopian novels to be sci-fi, so my options are somewhat limited. I suppose I'll have to say Douglas Adams even though I consider that his books are more humor/commentary on human stupidity because the other options don't really work - Lem probably gets honorable mention.

On fantasy, I'm tempted to say C.S. Lewis because I realize that I'm tempted to re-read Narnia lots of times, whereas no matter how much I enjoyed Tolkien and Rowling, I don't feel that same pull to re-read. Lloyd Alexander probably gets honorable mention. Martin is in the shitter in my estimation because I think he's let the series get away from him. Sanderson is fun but his prose really needs some improvement.

My favorite author overall is probably a tie between Dostoevsky and Bulgakov. They have completely different styles and Dostoevsky is considered much more of a serious writer, but Bulgakov's works are just...wonderful. Hence the tie. If you had asked about my favorite book, I'd probably say Pasternak's Doctor Zhivago, but Pasternak was first and foremost a poet and Zhivago was his only major prose work (or at least, the only one worth reading).