Arial is the best font for Email Signatures for a simple reason that it is readily available on all systems. Using fundo images or HTML signatures is not advisable in EMails as not all EMail clients will support images or HTML.
Though Calibre is the recommended font by Microsoft Outlook, if you are not using Vista or Office 2007 then the font may not look the same on all systems. So it’s best to stick on to simple black fonts to make sure your message goes through clearly.