Hey, I know I'm new, but hey, thought I'd throw in my 2p! I'm going to leave off a few that have already been mentioned (ie LOTR, HP, Earthsea) to save repeats!
Raymond Jaques, Redwall
Have only read the one, and it went on a little long, but still well constructed.
Phillip Pullman, His Dark Materials
I absolutely love these. Similarly, The Firework Maker's Daughter is great.
Eion Colfer, the Artemis Fowl series
Currently on the fourth, The Opal Deception. These combine sci-fi and fantasy usually with a bit of sluthing in there too. Aimed at the 9-12 audience, but it means I can read one in a couple of days worth of bus journies!
Darren Shan, The Vampire Chronicles and Lord Loss
Again, aimed at 11+, easy to swallow sections of a long and involved tale or two, with brief flashed of the most horrible and in cases heartbreaking violence along with life-lessons for boys. A great all-round read for any usually reluctant young males you might know!
The Chronicles of Narnia
Became disillusioned with them after I did a research project on C.S. Lewis and found out about all the Christian "undertones", but they are still very short, very perfect stories, showing that you don't have to have a minimum 400 pages to capture the imagination (Ms Rowling!)
As I have said, I'm a Primary School teacher, so I read a lot of things that children I have access to have access to, though I have brushed with Gaiman. A lot of my adult reading right now is to finish my degree, for which the exam is "Feminist Writing", so it's plenty of Angela Carter and Margaret Atwood for me!