(All of the following are macos only). I've used OmniOutliner, NoteBook (Circus Ponies, now defunct) and Growly Notes (www.growlybird.com). I've enjoyed using Growly Notes for many years, first as a free app and now as a nominal $4.99 on the App Store. It is closed source, but it's very versatile, flexible, and has a good scalable organization heirarchy (notebooks/sections/pages). It can import and export many different formats. It's also currently maintained and the support is good.