I think my favorite book in the series was probably between the Third one and the Fifth one as I liked the stories told in both of those a lot. The last was well paced I thought as well, and I really enjoyed the ending as I found them both (there is one that King himself says that he didn't really want to write, but did for those people who would want closure on certain things) very well done.
My favorite character had to be Roland as I always found myself wanting to be seeing things in his POV when I was in the other characters. He was very real in every aspect and loved his weaknesses.
Not like I didn't like the other characters, but I felt for Roland and his overwhelming desire to complete his quest for the tower.
A thing that I really enjoy about the King books is his world building and this one, along with "The Stand" is another great one. To the small things like the way that Roland pronounces certain words, to the bigger issues, like the varying religions and beliefs that run throughout the world. It was very easy to get sucked up into the story and world.
Next up for me is "Gone" by Michale Grant. I'm looking forward to it as I think the idea of a world without parents very compelling. Look for my review in the next few weeks.