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.)