I don't think there is a way to mark an email as a default. I think Gmail sorts the email initially based on the Contact Name and then on the actual Email Address.
For instance for a contact name Barry O'Connor with one email as barry.oconnor@sin and the other is barryoconnor@gm (I haven't put the whole email address here)
If you type barry in the To: box it will display both email addresses. Type an . or a an o it will filter down to one email address.
If you type the full contact name Barry O'Connor then you have to choose which email to use from the box.
I would have uploaded images to explain this but there seems to be a problem with Imgur.com at the moment.