Apple - In Mail.app, how can I set a recipient's default email address?

You can if you have the contact in a group in Address Book. Select the group, then go to Edit > Edit Distribution List... You can select the contact's default email address (the default one is marked in bold). As far as I know, this only sets the default address when emailing that group, though. I don't know of a way to set a default address for a "normal" contact. (After trying to figure out a way off and on for several years, I'm inclined to say it isn't possible, but hopefully someone will prove me wrong.)

Remove the email address you don't want to be the default from the 'Previous Recipients' list in Mail.app:

Mail -> Window -> Previous Recipients -> Search for Name

This will make the default (and much faster) first address the one in the Previous List and any others in the address book will be further down the auto-complete list.

This works until that recipient sends you a new email from that same address, which puts that email address back into the queue.