My favorite science fiction writer is Ursula LeGuin, and my favorite of all her books is The Left Hand of Darkness. It's one of her earliest novels on the Earthsea idea, and is captivating from beginning to end. LeGuin doesn't proselytize or preach; she simply presents a wonderful, alternate universe and makes it plausible. That's quite an art.
I had trouble sorting out my favorite stories, since I prefer science fiction that is subtle, with an undercurrent
social message. Mike Resnick is an author with a message, but his stories are a blur in my mind. Among the classics, I want to mention Karel Capek, an author who wrote R.U.R., and whose book ushered in the use of the word "robot" into the English language. I read that book in my teens, and it has stayed with me.
Capek's philosophy resonates with me, and one of his lines is worth quoting here:
"Much melancholy has devolved upon mankind, and it is detestable to me that might will triumph in the end ... Art must not serve might."
He died in 1938. In all of his work, Karel Capek warns against man's misuse of science and his desire to master life.
Both Ursula LeGuin and Karel Capek are writers who deserve to be read again and again and again.