Sci-fi only? I'll throw in a couple.
Lois McMaster Bujold's Vorkosigan series. I advise starting chronologically with Shards of Honor. And please ignore the later-released "Harlequin Romance" omnibus covers, I don't know WHAT the friggin' publisher was thinking
C.J. Cherryh's Chanur Saga, starting with The Pride of Chanur. Her grammar gets a little choppy at times, possibly as an artistic decision, but that's the only imperfection. What a great story, and more, what a great twist on a concept that's been done to death.
Fun books
Keith Laumer's Renegade's Honor. Middle book of the series and the only one I've read, but great classic space opera sci fi.
David Weber's Honor Harrington early books, the first three or 4 were great.
David Weber's Empire of Ashes series - Mutineer's Moon etc. Nice work and a great look at some things that might be in our near future.
Michael Stackpole's Battletech books - surprising lot of political intrigue and character work for a book about giant robots. I still think this universe is an insanely good fit for an MMO too.
...I don't usually recommend Star Wars books, buuuut - Shatterpoint by Matthew Stover was well written and a surprisingly good read, well above all of the others I've read.