Hi,
i just want to bring a font to attention, if it is really the best is a personal choice, of course.
The default i guess is Courier, but i for myself like Source Code Pro more. It's a monospaced font, totally free and distinguishes clearly 1iIlL| or 8B or oO0.
Of course this is not really Sigil related, but maybe useful for the one or the other.
You can read more about this font here:
http://blog.typekit.com/2012/09/24/source-code-pro/
and download it here
https://github.com/adobe-fonts/source-code-pro
//rubeus