Well, vi, since it's part of POSIX. Unless you are fine with ed ;)
[0] https://en.wikipedia.org/wiki/Nvi
On one of my early jobs I had to telnet to an AIX machine to do some work and, of course, joe was not installed and vi was.
I had almost no idea on how to use it, and by the end of that project I had decided that I needed some vi skills!