If you are looking for cross-platform like MacOS, iOS, Android (I am not sure about Windows) then I recommend https://ia.net/writer/ It has apps for iOS and Android and app for MacOS. It is very fast (at least this is my experience) and supports sync with iCloud and Dropbox and Google Drive. I use only iClod and it works amazingly.

I am using it for over 3 years I think. It just supports Markdown simple. You can choose your extension .md or .txt - depending on what you want to do next with your files.