Assuming you fly in cheap economy and you want those miles for award flights later, then AA is your best choice.

Regarding the validity: As FedUp2 said, any sort of activity stops the miles from expiring.
So just an online promotion, transferring points from a Starwood or anything will put more life into your accounts. So you just need to make a note in your calendar to keep track of time and you can keep those precious miles alive.