Solving a problem. Pondering on a problem of plot, character, the world and the rules that govern it, reaching that point of desperation and thinking I'm just never going to come up with a way to solve it... And then I do. There's nothing like that moment. The "Aha!" moment or whatever they like to call it. And the way everything just fits into place afterwards? Amazing.
This is exactly what I love also. I can be stuck for weeks trying to make sense of things. My problem is I always come up with something to fix my story at 3:00 am. I wake up and it just hits me. I sit up in bed and scream to myself "You Fool!" Then I can't get back to sleep until I write it out. Many times I would find myself falling asleep at work because of this. But there is truly no better feeling for me when I work these problems out.
Probably the initial idea and initial stretching it out into a rough outline. That's before the hard work begins.
Worldbuilding. If I ever do become successful enough to warrant a book just about one of the world's I've made, then it will likely be larger than most people care for.