Okay, if you are looking for series...
First, the standard against which nearly all fantasy is compared: J.R.R. Tolkien. The Hobbit is a children's book, but almost a mandatory read before you tear into the Lord fo the Rings trilogy. Similar series in the Fantasy Genre (and which have all been compared to LotR) include Terry Brooks' Shannarra series, and Stephen R. Donaldson's Thomas Covenant books (Donaldson has some pretty good science fiction, too. I just finished reading his Gap Cycle series).
Brandon Sanderson's Mistborn trilogy is also good, and of course there is the now-ubiquitous Potterverse of J.K. Rowling.
In sci/fi, there are the military books like the aforementioned Vorkosigan series by Lois McMaster Bujold, which I have yet to read myself, but it is highly regarded, as well as David Weber's Honorverse and Elizabeth Moon's Serrano books (Moon also has a great fantasy series, The Deeds of Paksenarion).
For hard sci/fi, there are a few standalone books which I like: Bones Burnt Black by Stephen Euin Cobb, and []The Martian[/i] by Andy Weir.
I like everything C.J. Cherryh has done, though I am a few books behind in her Foreigner series. I also read anything put out by C.S. Friedman.
Mind you, this is only the tip of the glacier... I can make more specific recommendations if you are into certain genres, such children's literature, hard sf, space opera, steampunk, super-powered fiction, urban fantasy, non-fic, etc.