I would echo the earlier recommendation for Alan Moore, although he is mostly in the comic field. Neil Gaiman is amazing, but I wasn't a huge fan of American Gods. Stardust, Neverwhere and Coraline are his best. Gaiman himself would admit to being heavily influenced by Angela Carter (Nights at the Circus, the Magic Toyshop, the Bloody Chamber) and Mary Gentle (Rats and Gargoyles, the Architecture of Desire, Ash).
I agree with you about the Harry Potter, although I have read them all. In the end I just wanted to get the darn things finished. But I wouldn't write off the younger reader novels. Both Philip Pullman (HIs Dark Materials) and Philip Reeve (Mortal Engines) are excellent. I would also recommend Lian Hearn (Across the Nightingale Floor).