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.
Out of curiousity, is there a more ideal format for import/export?
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!