I'm glad that question's in the plural form. In sci-fi I'd have to pick "Ender's Game" by Orson Scott Card, "The Mote in God's Eye" by Larry Niven and Jerry Pournelle, and "Genesis" by W.A. Harbinson, just based on how many times I've read them. Ask me again tomorrow and it'll be different. For non sci-fi fiction, "Last of the Breed" by Louis L'Amour really blew my hair back. Despite what you might expect by the author it's not a western. It's about an American military pilot of Sioux ancestry who crash lands and is captured in Soviet Russia and has escape and trek his way out. My favorite non-fiction, judging by how worn it is alone, "Surely You're Joking, Mr. Feyman" about the crazy adventures of Richard P. Feynman as told to Ralph Leighton.