Apple Reminders, which I now keep in a widget on my phone & iPad home screens. This is mainly for repeating items, like shopping, since I can turn on “show completed” and then uncheck them to put back on the list.
Or paper notebook, which I normally have in my pocket. This is for more serious things where I need to write some procedure or notes.
Used to use Things, which is great, but it’s overkill for my current needs.