I still have my email stripped down to plain text for display, and I send my email as plain text. So I use fixed-width fonts. As a long-time Mac user, I like Monaco (or Montagel) for this.

The best is to send as plain text so that the recipient can see your email in the font of his choice. Don't you prefer to see incoming emails in the font of your choice?

Not sure if you mean what font to use for outgoing mail, or to view incoming mail. Not much point in using a non-standard font for outgoing mail because if the recipient doesn't have it, a different font will be substituted.

I send all mail in Gmail with the default, "sans" (which is Arial). I rarely try to "design" e-mails, and even when I do, it will be through layout, not type choices.