Can't pick one, same way you can't really tell which child is your favorite. And I hate them and love them at the same time! First drafts, for instance, are so liberating because the rules of the world aren't set yet. Revision is exciting because now you know where you're going and you can be deliberate about your dialogue and ruthless with the stuff you thought you wanted to stick. Action scenes are harder for me, and sometimes my dialogues go nowhere in first drafts. But injecting tension in everything--I love that. Suddenly you realize why this word is here, or why it doesn't work, or what you need to change. Each part really has its own thing going on.
The cause of all things bad is a lack of imagination.