NaNoWriMo has always been my most successful writing, if by successful you mean word count and time spent. Three years in a row, I’ve yet to fail to hit my 50k words, even if I’m not quite at “The End” by the end of thirty days.
It’s also successful because you don’t have time to edit. You don’t have time to second guess yourself, or to worry about whether you’re putting in enough subtext, or if you correctly referred to one thing or another. You just have to get the words out, and you don’t worry about quality. That’s a freeing thing.