Personally I find Apple Notes the most convenient for quick jotting down of ideas etc. It’s not the most advanced in terms of features but it loads instantly, is quick to use and is available on all of my devices. Have tried numerous alternatives but they’ve never stuck e.g. they’re slow to start up or are fussy to use or don’t work well on one platform.
For deeper sketching out of ideas, diagrams etc I really like Concepts on iPad with the Apple Pencil. It has an infinite canvas and is all vector based, which is great for never worrying about whether there’s room to fit your idea on the page or whatever. Previously I used Notability and it was good and a bit more traditionally note based, but I miss the infinite canvas. The text recognition and handwriting features in Apple Notes on iOS 14 are pretty cool though, will be nice to see what third party apps do with them.
For deeper sketching out of ideas, diagrams etc I really like Concepts on iPad with the Apple Pencil. It has an infinite canvas and is all vector based, which is great for never worrying about whether there’s room to fit your idea on the page or whatever. Previously I used Notability and it was good and a bit more traditionally note based, but I miss the infinite canvas. The text recognition and handwriting features in Apple Notes on iOS 14 are pretty cool though, will be nice to see what third party apps do with them.