I tried tons of (free, non-subscriptive) To Do Apps incl. Wunderlist and Producteev is by far the best, with the biggest feature set and user-friendliest UI.
Yet it lacks some important features as e.g. better/deeper integration in iOS (e.g. "Send to", "Open in" in iOS context menu, Siri-connectivity), integration into calender (as e.g. in Wunderlist), maybe a bit sleeker and more efficient workflow for creating new tasks incl. setting of due dates/reminders, and the possibility to change notification sounds.
Also, I experience stability issues from time to time, especially on the iPad (2nd Gen).
Apart from that, a super-productive app of great use for no money - highly recommendable for everyone whos trying to get organized in business, social, family and personal affairs! Theres literally no other app which might beat that.