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Too many to count. I have about 500 songs my ten top favourite songs, same with books.
Day of the Triffids - John Wyndham (plus all his other books)
Nightfall - Isaac Asimov (Ok everything by Asimov)
Dune - Frank Herbert
Martian Cornicles - Ray Bradbury.
Riverworld - Philip Jose Farmer
Anything by Larry Niven.
Everything by Terry Pratchett (not strictly sci-fi though)
Loved Brave New World (& the old movie + new TV series).
Then there's Arthur C. Clarke, Harry Harrison, Eric Frank Russel, Robert Sheckley to name a few.
The Foundation series (the original 3 books, a bit less for the other prequels and sequels).
Dune (the original 3 books, obviously, and a bit less for the other prequels and sequels)
The Lost Fleet (Jack Campbell)
And I know, unpopular and controversial because of the Author and what he stand for (will remain nameless and no link provided), but they are fun and entertaining....
Battlefield Earth (the movie was horribly bad) and Mission Earth.
I am ashamed to admit I read the whole damned thing, thereby wasting a couple of weeks in my reading life. At that time I always finished a book, even when it was awful. I could have been reading something more entertaining, like used toilet paper.
Having +/- 1200 of them it is very hard to choose ( I know, I am an addict ) but I did enjoy the Foundation series, the Dune series, The dragons of Pern and many others. Special mention though for Douglas Adam's and Terry pratchett's stuff. After all it is very hard to beat 42 and the idea of a discworld.
I've got around 2,000 now, and that's after heavily winnowing my book collection over the last 20 years. When we moved into our previous house, the moving company added a surcharge for the 250+ boxes of books my wife and I had between us.