Depends on what kind of sci-fi you're looking for. I'll list authors and books/series for a good entry.
Hard Sci-Fi[More on science]: Larry Niven [Ringworld], Robert Heinlein [Starship Troopers], Issac Asimov [Gold], Arthur Clarke [2001 Series].
Soft Sci-fi[More on character]: Harlan Ellison [Vic and Blood, the 3-part Boy and his Dog Collection], Theodore Sturgeon [Saucer of Loneliness, or More Than Human], Spider Robinson [Callahan Chronicals, the first three books in the series], Philip K Dick [We can remember it for you wholesale, Minority Report, Do androids dream of electric sheep?].
Wartime Sci-Fi[Space battles, pewpewpew]: John Ringo [A Hymn Before Battle], Sandy Mitchell [CIAPHAS CAIN, HERO OF THE IMPERIUM!], Dan Abnett [Gaunt's Ghost series, Ultramarines series], Robert Heinlein.