I agree with most of the above, particularly the five Foundation novels. Also on my must read list:
Anne McCaffrey (and later son Todd McCaffrey): Dragonriders of Pern, The Ship Who Sang et al, The Tower and Hive series (The Rowan, etc. - named my daughter after the main character), Doona and much more.
Ursula K. Le Guin: Earthsea, Hainish Cycle (particularly The Left Hand of Darkness).
Andre Norton: Beast Master (the movies were a joke), The Solar Queen series, The Time Traders & Witch World series, Voodoo World, and a quirky little surprise called The Sioux Spaceman (trust me...).
Marion Zimmer Bradley: Yeah, she writes sci-fi - The Darkover series, for one. But don't skip the Avalon Series - or you'll kick yourself.
...and if I were king, NOT reading The Martian Chronicles and Stranger in a Strange Land would be punishable by 20 slaps with a wet cod.