Foundation Trilogy. While I read about half of Asimov's fiction books, the original three-volume Foundation is the best SF series I ever read - I own two English-language versions, the nerfed German translation of the first edition, one decent (more recent) translation and the best translation of them all, the one by Heinz Nagel.
I also liked Saberhagen's Berserker series (though that's more scifi* than 'serious' SF, Vance's Durdane and Alastor series (though Vance hardly writes 'hard' SF), and Cordwainer Smith's Instrumentality series. Hubby is more into Cyberpunk stuff, but that's too close to reality for me .

As for stand-alone books: I happen to love Emphyrio by Vance, Mindgames by Cadigan (eventh tough it's Cyberpunk), Heinlein's Starship Troopers and Number of the Beast, and 'The Mote in God's Eye' by Niven & Pournelle (though I can't stand the books each wrote by himself, if for different reasons).
Apart from those, there are gazillions of SF books I like, but these were the first to come to mind. SF was my favorite genre when I was still reading a lot (i.e., had more time).

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*In my world, the handle 'scifi' is reserved for cheap TV series and pulp fiction.