Vi
The original Vi editor was written by Bill Joy in 1976. It has been ported to just about every operating system that exists. I have used this editor for many years and really like it. Why is that? Well, for one thing, it's very efficient. You can move around in your document with very few key strokes in a very efficient manner. No mouse required. You never need to move your fingers away from the keyboard. Also the editor works great in any kind of terminal or terminal emulator that supports basic cursor control. So you can ssh into some remote host and do all your editing very easily.
You do need to memorize some basic key commands and also remember that this is a modal editor. But once you play around with it a bit, Vi becomes a real pleasure to use.
Here are a few sites where you can find out more than you ever wanted to know about this classic editor.