There are some great super-niche notes apps designed for fiction writers or developers, but they aren't fit for most people, so weren't in consideration for a place on this list. Similarly, there are lots of powerful business collaboration tools that claim to be notes apps but just aren't suitable for one person.
I also only really considered general-purpose personal note-taking apps.
So for this list, I didn't look at every app that could be used to take notes or even every business collaboration tool that claimed it was a notes app-I was only interested in apps that were explicitly designed to be notes apps.