Oh, that's a tough question. There are so many things I love. A close second would be hearing from my readers about them enjoying my books, but I'd have to say that top of the list is finishing a book and looking back on it.
What I mean by that is that a book takes a long time to finish, and it's easy to lose perspective. I can forget that the ideas, people and places in the book didn't exist when I started. I didn't know any of them.
It's a pretty substantial feeling of accomplishment to have created a new world, new people and (hopefully) interesting things that happen to them.
Plus, as a reader, it's a big piece of my lifelong dream to be an author, so it's nice to publish a new book and fulfill that dream.