To keep track of tasks I want to complete I´m using the Reminders app that comes with MacOS and iOS. The first thing I appreciate about Reminders is the synchronization across all Apple devices I use. You can take it for given, but still, I think it is an important and well working feature. I use the Reminders app in a basic fashion, which means:

The app supports working with those lists in an excellent way on all devices. I do not use the fancier things, like tags, smart lists, subtasks, sections, and what else there might be. Apple Reminders is a powerful app. It´s much better than what I would expect from an app that comes with the operating system.