Hard Question. Used to be Stephen King...but I think he's loosing it a little.
I like Neil Gaimen..
http://www.neilgaiman.com/index.asp
He writes really interesting fantasy sci-fi stuff. He also has a couple kids books (Coraline, THe day I swapped my dad for 2 goldfish, the wolves in the wall).
My BF introduced me to him when we first started going out. He lent me a book called "Star Dust". It's an illustrated Novel about a fallen star. Very cute.
Other books of his I HIGHYLY recommend are
Neverwhere (about a secret underground world in London),
American Gods (about how when people worship an idol, they become real, and about the son of a god..Odin actually, hence our dogs name, and how he gets involved in a war of the gods), and
Good Omens (Hilarious, about the 4 horsemen of the apocalypse, and how the child of the devil, the one who will bring down civilization, is switched at birth).
Neverwhere was made into a movie..not that great though.
There is a movie coming out which is a joint effort between Neil Gaimen (writer), Dave McKean (drawings) and Jim Henson Studios, called
MirrorMask. Here is a link to the preview.
http://www.sonypictures.com/previews...-p7119865.html
(click on trailor in upper left side)