I love the Dark Tower Series, I've finished the first 5. For others who enjoyed it, don't miss The Talisman and The Black House, which are Stephen King/Peter Straub collaborations and tied into the Dark Tower books.
My favorite...that's a tough one. I loved Bag of Bones, The Stand, and IT. Also, another one that a lot of people miss is Eyes of the Dragon (?) I think! It's really great, fantasy, evil warlocks, etc.
I absolutely, totally, completely abhorred Dreamcatcher. I was so disappointed in it I almost cried.