I don't have just one. Under Windows, I use a fork of Florian Ballmer's Notepad2 to replace Notepad. It's one of a number of editors based on Neil Hodgson's Scintilla edit control. It's small, starts fast, and can replace Windows Notepad through a registry hack. I also have Notepad++ and an assortment of other things installed.

Under Linux, I use Vim, emacs, and Geany, which is a lightweight IDE also based on Scintilla.