I love the feeling of being deep within a story, of watching as character arcs and plot arcs twine together and mesh exactly as they should -- and even the feelings of surprise when something unexpected (but perfect for the overall book) pops up. This is my favorite time of the writing process.
My least favorite time is the first draft. When I have a truly viable story, the entire thing sits in my head, percolating. I try to wait to write it down until it's settled and has a clear beginning, middle, and ending. Then, depending on how much time I have each day to write, I can usually get the bare bones of the story down (almost all plot, btw) in a couple of weeks. The fastest I've ever done this (because I happened to have a ton of time) was ten days. I think I move so quickly because I dislike that part so much.
But once I have the basics down, then I get to think more and watch the characters for a bit in my mind. I pretend it's a movie, watching how they interact and change. And I send it off to my wonderful (truly wonderful) critique group who always catch the inconsistencies and ask the best questions about motives, things which happen off stage, and previous lives. Their thoughts get my imagination stirring, and then everything starts to really come together. I love how one thought/question/comment can unveil hidden depths -- and perhaps change the entire direction of a character's role in the book.
So the end is in sight; the characters have deepened and their story has grown. I still have a few consistency tweaks to deal with, but for the most part, I think everything is strong and ready to go -- now I have to gear up for the querying process...which actually is my very least favorite part of the process.
What's your favorite step along the writing journey?