I may be internet famous, but I am not a primary source of original research
Yeah, I remember my teacher gave me a 10/10 on that one. 15 years ago. But seriously.
(Thanks to Eduardo for pointing out what happens when you search for "apenwarr" or "Avery Pennarun" in Google Books. Answer: silly things.)
ssh+2FA to all your machines, anywhere, without opening firewall ports.