If you've dismissed any of these based on your reading as a kid, try re-reading. I read Tolkien at 12, loved the wizards and orcs, but as an adult remembered it as nonsense. When I re-read it (to be able to discuss with my 11 year old son), I was pleasantly surprised at how good it was. Likewise the Dune books (all but the first are probably beyond any adolescent). Good fiction is complex, and readings fascinated with wizards and swords and knife-fights will miss a lot of thought-provoking stuff. Nothing wrong with a good wizard, but don't miss the rest.

That said, I'm sorry to say that the Zelazny books, especially the later ones, don't withstand re-reading. And I don't see how "Old Man's War" appeals to any one but a teen or an ideologue. (I share the ideology! but the writing is stale as yesterday's cornflakes, and the characters as unidimensional as Heinlein's laziest pulp, without the Heinlein all-transgressive idiosyncrasy.)

Last note: "Starship Troopers" and "Forever War" ought to be read in sequence. I can recall reading Haldeman at 15 and thinking "Hey! He's going after Rico's Roughnecks!" It was my first moment of literature as Great Conversation. And as I read some political philosophy, I could see Haldeman as a Chomsky / Rousseau "it's all socialization!" response to the Locke / Hobbes rationalism of Heinlein. It was an early experience of fiction as thought, and a suggestion of why philosophy alone won't do.

Now I gotta go read the stuff on the list that I've missed.