Question: to do list app with subtasks?
Answer: ClickUp. In most of the apps listed here, you'll have to pay extra for things like notifications, managing tasks or assignments. ... 
Todoist. Todoist was recently named the best to do list app by The Verge and it's easy to see why. ... 
Remember The Milk. ... 
Google Keep. ... 
TeuxDeaux. ... 
Things. ... 
Wunderlist. ... 
TickTick.