Ranked in these QuestionsQuestion Ranking
Pro 100% Free
Just like todo.txt cli, this is 100% free (although donations are accepted).
Pro Also comes in a cloudless an Owncloud version
For those who don't care about syncing there is a cloudless version while people who run Owncloud may opt for the Owncloud version.
Pro There is an add to calendar button
This not only tells one's calendar (and thus any devices that sync with your calendar) about the todo, but allows all synced calendars to play an audible alarm when the todo is scheduled to be finished.
Pro Seemless compatibility with todo.txt cli
Let's one sort to-dos by due date then priority
Con As far as I know this app is Android only
I believe Simpletasks is an Android only app.