The editor: http://www.crimsoneditor.com/

The reasons: It highlights syntax in most forms of code including HTML,PHP and CSS; it has line numbering; it uses tabs; best of all it's free!!!

I used notepad for many, many years and finally got sick of trying to find single /'s or <'s etc that I might have inadvertantly missed. Crimson edit highlights the syntax as you are coding it, so you know where you have missed code as soon as it happens.