*Browses bookshelves*
I'm restricting myself to less well known authors (although some of these are borderline famous now), so take Harry Potter, Old Kingdom, Dark Materials etc as implicitly recommended
Fantasy- Trudi Canavan - I prefer her Age of the Five trilogy, but her Black Magician Trilogy is more famous
- Sergei Lukyanenko - Night Watch Quartology (Much better than the film 'adaptions'
- Brandon Sanderson - Mistborn trilogy, although his Elantris, Warbreaker and Way of Kings books are also good - WoK is the start of a 10 book series though. Recently found fame as the author completeing the Wheel of Time.... speaking of which
- Robert Jordan - Wheel of Time (Although I suspect you'll have to settle for the first book, which is fairly standard Tolkeinesque unlike later books in the series - as I don't think your budget will stretch to 13 books).
- DJ MacHale - The Pendragon Adventure - standard YA fantasy, with a fairly good overarching plot which kicks into high gear after book 4. Although the last book left something to be desired. Everyone like a good YA book once in a while right?
- Ursula K Le Guin - The Earthsea Quartet - certainly a very good quartet, better than the anime adaption. Available as a single volume.
- Terry Brooks - The Word and The Void Trilogy - highly recommended, less well known than his other series Sword of Shannara, the latest book of which tied the two series together. Available as a single volume.
- Terry Goodkind - The Sword of Truth Series - The first book is quite good, and has been 'adapted' into a TV Series in the US recently. After the first book it does start to ram his personal Philosophy down your throat so hard it can come back up and onto the book though... Sometimes said to have ripped off, and made worse, Robert Jordans WoT series I previously mentioned. Make of that what you will.
- Natasha Mostert - Season of the Witch - debateably Fantasy, although also fits nicely into Thriller (and Romance to an extent). Actually, not sure what genre this fits in, but I found it a wonderful read that stayed with me for a very long time.
- Suzanne Collins - The Hunger Games - YA Fantasy Trilogy, that is actually very gripping.
That was slightly longer than I imagined it would be, and I still have sci-fi to go, although that will be noticeably shorter. They aren't in any meaningful order, just as I saw them on my bookshelf. I haven't listed Twilight, as I'm fairly sure I know the reaction I would get, but it is good for a mindless escapist read.
Sci-Fi- Alastair Reynolds - Revelation Space Pentology - Hardcore SF. Not for the faint hearted.
- Geoff Ryman - Air - A very good light SF novel, examining the effects of Technology on a remote village that's not ready for it.
- Gene Brewer - K-PAX - Someone who claims they're an Alien is committed to a psychiatric hospital. The book series leaves a very different feel from the movie that was adapted from the first book. Very much worth a read, even if you have seen the film.
- Rob Grant - Colony - Humourous light Scifi about a mission where over many generations the crew become idiots, and revive a member of the original crew. His other books are also very good, although not Sci-Fi or fantasy.
I said it would be shorter, didn't I?

And finally, one which is arguably every literary genre under the sun that more people must experience (not read, but experience) is
House of Leaves. (Yes the blue is deliberate.)
EDIT:
OK, Just read one which you absolutely must stock.
Rod Rees - The Demi-Monde: Winter. Steampunk + Virtual Worlds + Vampires + Robespierre (Instrumental in the French 'Reign of Terror'), Shakah Zulu (Creator of the Zulu empire), Ivan the Terrible (Essentially laid the groundwork for the Russian Empire), Empress Wu (only woman to ever rule China), Heydrich (Mastermind of the Holocaust), Beria (head of Stalin's secret police), and various other psychotics from History all together in the same place...