1984 is one of my all-time favorites, especially because it highlighted the dark side of a technology we rarely think of as such: language. It’s that kind of astuteness that I’m looking for, I suppose. (Loved Brave New World, too.)
I read Ender’s Game around age 13, at the encouragement of a couple SF-loving friends. I recall really enjoying it (and it seems it’d be even more relevant today given the emergence of drone-based warfare).
I saw the film adaptation of Flowers for Algernon around the same age and over two decades later I still have yet to read the book, which I’ve been told (surprise) articulates the story’s message better than the film.
As I mentioned in a thread discussing Babylon 5 (which spurred me to create this thread) I’ve also read and freakin’ loved the Hitchhiker’s series but I consider that comedy–dark, glorious, hilarious comedy–rather than sci-fi.