Question: web app to do list?
Answer: Todoist (Windows, macOS, Android, iPhone, iPad, Web)
Todoist price: ... 
TickTick (Android, Windows, macOS, iPhone and iPad, Web)
TickTick price: ... 
Microsoft To Do (Android, Windows, Web, iPhone and iPad)
Microsoft To Do price: ... 
Things (macOS, iPhone, iPad)
Things price: