You may want to consider TiddlyWiki. It’s a single HTML file that you can sync to all of your devices.
Another option that I like is Safe Notes on Android (I’m a bit paranoid) which encrypts your note files on the phone and then uploads to protectedtext.com. You can then edit those notes on the web with your encryption password. No account is needed — you just go to the URL of your notes (e.g. https://protectedtext.com/uketabs) and it’ll ask you for your password. Go ahead and click the link — It’ll let you create that page. Any changes you make on the web you can download/sync back into the mobile app (with the password, of course). I dunno, it’s nifty I think. The downside is that there isn’t any formatting…it’s just text. But, I feel that’s a good thing because you’re not locked into any proprietary file types or services.