I keep going back and forth between Checkvist (awesome for outlining and planning!) and Todoist (habit-forming).
The only thing that keeps drawing me back to Todoist is the ability to track # of tasks completed by day (and week). Keeping up a "streak" of 5+ tasks per day, though they are of all different shapes and sizes does something psychologically to keep me engaged with my organization system. I could of course just tally my tasks in a .txt, but it would be nice to be able to do it right in the organization system itself. Complete engagement.
Checkvist is way more my style with the keyboard-oriented setup, the awesome pro options, and the very functional, well-outfitted free version. But I keep getting drawn back to Todoist because of the habit-forming nature of it!