Turtl: https://turtlapp.com/
Largely because it's open source and privacy oriented, encrypted client-side. It's not as convenient and feature-rich as OneNote, but entrusting Microsoft with all my notes seems crazy.
I love Turtl's interface, tagging system, hosting options, multi-OS setup, and privacy stance. Turtl would be my go-to solution if they supported .md import/export. I think they have .json export planned for the next release, which might be tractable with a bit of pandoc manipulation, but not ideal.
I appreciate your excellent work. Google Keep, for example, exports both a .json and a folder of individual html files. I ended up piping the individual .html files through pandoc to .md files for idiosyncratic reasons. To be fair, I think turtl is so many leagues ahead of other solutions on the rest of its feature set that a .json export - which appears to be standard - is enough.
I suspect that turtl appeals to a lot of users with a large directory of plaintext notes who would be enticed to adopt the service if bulk textfile import were also streamlined. I hope the next release is a success!