Well, there are a few, but based on how I currently feel, I'll kinda rank them:
1. Artemis Fowl (And sequels) by Eoin Colfer. I actually own one of the first versions of this, where it looks like the "Book" rather than with the cool cover art. I like all of them (haven't read The Atlantis Complex yet), but I think my favourite is the original, with The Lost Colony and The Eternity Code coming closely behind. Although I haven't read the latter for years, as I don't own a copy. I guess these are sort of fantasy novels really.
2. Alex Rider series by Anthony Horowitz. Impossible to separate these out, although Eagle Strike, Scorpia and Ark Angel are probably my favourites, as he's not working for MI6 in those, which makes it more interesting as he doesn't have a set goal or anything, he sets himself to do things, still spying even though he didn't originally want to.
3. Inheritance Cycle by Christopher Paolini. Love these books. Ironically, it was my brother who bought Eragon and Eldest, yet I seem to like them more than him, so I bought Brisingr (which is definitely my favourite) and plan on obtaining the final book too. These are definitely fantasy books as well.
4. Power of Five series. Another Horowitz one, these are possibly the darkest books I've ever read, although they're not really dark, but I still like them. Eagerly awaiting the 5th book, as well as hoping to eventually buy Raven's Gate. Don't really know what genre these are...
5. Harry Potter series. Yeah...I've read them all. Seen films 1-6, own films 1-5 on VHS and DVD too. Don't get me wrong by ranking them so low, or dismiss them as a cliché choice, they're brilliant books. And the franchise on a whole is brilliant, I'd love it if Artemis Fowl or Inheritance had become as big as Harry Potter and been made into films.
10-ish. Alpha Force series by Chris Ryan. Another spy / adventure series, I've read them all once (or at least all that were released when I was really into them). They're really good, particularly Survival, the first book
All these are novels, because that's mainly what I read. However, I do read non-novel books, just less of them. Mainly Jeremy Clarkson's books, as they're hilarious.