A Post-It and a pencil, usually.

Not because "app bad" or "return to monke" or anything like that. Mostly because if I stow the note in a dedicated app, that somehow just makes me less inclined to write it down and read it later.

A scrap of papersticking out like a sore thumb on my desk or burning a hole in my pocket? I'm going to be cognizant of that all day long. But an obscure text file chilling in a disused part of my phone, or a txt file lost in the shuffle of random shit on my PC? Outta sight outta mind.

I also find all digital input schemes to be frustratingly less flexible than physical paper. Provided I have a writing utensil on hand that is functional (not always a given, granted) it is trivial to put anything I want on a note. Write anything I want. Draw diagrams. Underline or strike text. Write some things larger or heavier than others. All of these things are possible in note taking apps, but they come with the idiosyncracies of needing to know the selection techniques and menu options to activate them. In this way they're all death by a thousand tiny annoying cuts for me.

I even had a smart phone with a built-in stylus for a good long while. It definitely extended the things you could do with ease, but it was a far cry from a pencil.

The only thing a note taking app can do in my mind that paper can't is yell at you with a loud noise at a pre-programmed time. If I need one of those, I just set an alarm in my clock app.