I'd have to say it was probably "Dune" (and it's sequels) that did it for me too. I'd been reading SF and Fantasy for a while before that, but "Dune" really changed my way of thinking on SF. "Dune" was so much deeper, so much wider - the scope of it all was mind-boggling to me. That so much religious, philosophical and political thought was in there, waiting to leap out and change the way I viewed the world.

Only David Zindell's "Neverness" and "A Requiem for Homo Sapiens" have ever come close to repeating this experience for me.