The Dune series. Frank Herbert (not his son) Although the first book is my favorite, it might not rank at the top w/o the rest of the series.
The Moon is A Harsh Mistress. R.A. Heinlein.Wild political theories, a strange society & a revolution against an imperial power. what else could an American boy want?
Stranger in A Strange Land by R.A.Heinlein, I'll hold myself to two Heinlein novels.
Neuromancer by William Gibson. I'd pick Pattern recognition, but I'd feel the need to waffle over whether a new form of art makes a novel science fiction.
Snow Crash by Neal Stephensen, although The Diamond Age is close behind it.
Five, I'm not even positive this is my top five. I immediately ask what No Dick? No Anderson, Ellison, Harrison, Asimov, or Leiber? No Spinrad, neither Smiths, not a single Vinge, and nary an Adams. Where is Le Guin, Snodgrass, or Miller? Lists suck, you always underestimate the author who writes one great book for one who writes ten great books.