Canticle for Leibowitz was one that blew my young mind when I read it at around 12 or 13. Also the Foundation trilogy, and pretty much everything else from Asimov. Ender's Game was another. I'd forgotten the title of City and the Stars, but never forgot the awesome story. Rendezvous with Rama, another thought-provoker, as well as Larry Niven's Ringworld series.
I had to wait until I was 18, after discovering psychedelics, before anything matched the mind-expanding experience of reading those sci-fi greats of the 50's, 60's and 70's.