I've looked through AddressBook and Safari Preferences and could not find an obvious way. Internet search turned up nothing. One Discussion here suggested Autofill may use the 1st one in order, but it seems you tried that.
To change the email address that autofill uses while in Safari click on Safari>Preferences>AutoFill. Make sure the box is check next to "Using info from my Address Book card" then click edit to the right of that. Your Address Book card wil open and have check boxes to the right of you info. Check the items you want to use for autofill.
Hope this helps.
I had the same problems as above, info in autofill was being pulled from the wrong card. Could not change the card in contacts or on the mac.
It seems the solution is a mix of what WTBowman and Joe Gramm above mentioned.
First, log in to www.icloud.com..
*choose the card you want to be your contact card
* click on settings from the bottom left of the screen and choose "Make This My Card"
On your mac, open contacts and order the info (phones, emails, addresses) in the order you want it to be pulled with. The first item will be the one autofill will use to complete your forms.
I am using Mavericks 10.9