Hardly needing an explanation, Notepad is the simplest editor with the least features. It's been around since the beginning of time.
The notepad editor has almost no features other than the ability to type text.
Tips & Tricks
If you add .LOG to the first line of a text file in Notepad, every time you open the file, Notepad will append the current timestamp to the end.
If you show the status bar and turn off word wrap, you can see the line numbers.