I'm going to have to go with:
Dune: Frank Herbert
Altered Carbon: Richard K. Morgan
The Forever War: Joe Haldeman
Well, Dune has probably the most satisfying ending, ever. Altered Carbon has the best "rampage scene" in literature and the whole story is incredibly gritty and inventive. And The Forever War was just a great book, plus, yes. That is how the Army really works, they will fuck you in a heartbeat!
Crap, that's hard.
"Glory Road" by Robert Heinlein is always number one. At any given time, numbers two and three vary with my mood... as of right now, I'd say, perhaps:
"American Gods" by Neil Gaiman
"Foundation" by Isaac Asimov
My reasons? Well, for "Glory Road" it's just... I don't know. Jealousy. The Hero gets whisked away by a beautiful woman to go on An Adventure! Of course, the best part is after-- that's why I love the book so much. He saves the day, and then... gets bored. Eventually he tries to go back home, but he doesn't fit in anymore. So he goes back out, adventuring. It's been my favorite book since I was 15 or so.
"American Gods" is simply brilliant, and gets to me on an emotional level in ways I can't actually describe or even figure out.
And "Foundation" because it was my introduction to Asimov and to real sci-fi, and because I adore the Magnificent Bastards that are always Asimov's best heroes.
Foundation by Isaac Asimov
Stranger in a Strange Land by Robert A. Heinlein
Legion of the Damned by William C. Dietz