I'm tempted by the selections put out there so far, but I've got a couple of titles on my should read list. Both of these don't really need introductions. They've probably been read by everyone here, but I haven't and they are glaring holes in my required reading list.
My first nomination goes to Do Androids Dream of Electric Sheep? by Philip K. Dick. It's been on my should read list for years.
My second nomination goes to Brave New World by Aldous Huxley. I had a choice in grade 12 to read it or Charles Dickens. I choose Dickens. Looking back, I think I made the wrong decision. More than a couple of decades later, I've realized that I should correct that at some point.