The key for me is a) plain text files I can manage myself i.e. no database or mandatory custom sync; b) markdown.
Apps will come and go. You might decide to switch platforms and maybe a new editor will come along sometime. This means you want a format that can be opened by anything (plain text) but with lightweight markup that the editor can parse to make it look nice, but you can also parse with your eyes and get a reasonable sense of the document structure (markdown).
Then it's all about search. There's no point making notes if you can't find them. This is where something more than Markdown - that allows you to link notes - is handy. It's what is appearing more and more in the likes of Roam, Obsidian, etc.
I ultimately chose iA Writer on macOS because it is lightweight and really nicely designed, plus has good native support for Markdown. I sync using OneDrive but you can use anything because they're all individual files. iA Writer is also native, and I find most Electron apps to be slow and/or buggy. There are exceptions e.g. VS Code, but I prefer native where possible.