robbieduncan said:
That's not what he means

I don't think there is a way of doing what the OP wants. I think he wants to be able to say which email address on a per-contact basis is default (i.e. you have a work and a home address for each contact and you want it to default to work for some and home for others).
I could be wrong but it may be as simple as which one is first?
Aah... I see... now...
Need coffee... much coffee...
Lets see if this answer doesn't cover the question better: I would think you'd need to edit the order the fields on Card menu -> Add Field -> Edit Template... (or Address Book -> Preferences... -> Template) as the top field is default (as far as I've understood it), BUT, Mail also has a tendency to remember the last address you used for someone ** (but of course not always) so this will not be consistent...

...and I have a sneaking suspicion that Mail only presents different addresses in alphabetic order *** (when not found in the Previous Recipients), not the order you've put them in. Either way, no good for what the OP is trying to do...
** Edit: After a bit more snooping, my initial suspicion was correct: It seems mail presents the addresses as they appear in Previous Recipients sorted on date (but with those in Address Book first, and those not at the bottom of the list - some people I know have way too many email addresses

).
*** Edit: Again my suspicion was not too, far off, it seems that if nothing is found in Previous Recipients it presents the alternatives from Address Book, alphabetically, BUT not on the address field, but the name of the address field...
So, one solution to the OP: Make an e-mail category called aardvark (or something else that's first in the alphabet) then clear the (Window ->) Previous Recipients, and the next time you send an email to that group it will pick the aardvark address (I think)...
