The answer is Alaska Airlines. Reason: not so frequent a flyer that upgrades become an issue. With Alaska, you can fly Alaska to Seattle and also AA, DL, and some foreign airlines.

If you suddenly fly LGA-ORD weekly then maybe you should join AA or UA.

If you join AS, consider also joining either UA or CO so that your UA, CO, US flights will be covered.

To cover most of the other airlines, you'll also have to join Southwest and Jet Blue. There's also Air Tran.