Windows: Notepad.exe. Best ever, supports any language, can edit any text file, and comes as default on all computers, so I can develop on any computer which have internet access.

Linux: Nano. Easiest to use, and supports almost all flavours of linux.

@Louma vi is rubbish...
taskkill -9 vi

On Windows I prefer (if I have to) wordpad.exe. Ever opened a Config or some Source with notepad.exe? Even on Windows all IDEs and Programms (which are somewhat crossplatform) generate files with LF-Linebreak only. In my opinion that is good.
So does "CR LF" and significane of notepad.exe slowly fade away.

Try notepad++, so much better for windows! Supports a ton more!

And Mac supports Nano too! Nice!

TextEdit FTW, which automatically converts regular “s into curved and stylish ones 😂😂😂