Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Assuming you can find low-tier availability on AS' direct flights to/from SEA, then BAEC is best. Chase/BA card, Chase Sapphire Preferred card, Amex SPG card, or any Amex card that participates in Membership Rewards. Membership Rewards occasionally offers transfer bonuses of up to 50% when transferring to BAEC.
However, if you have any plans to travel to Europe or South America, don't forget about the free "gateway" stopover option on AA international awards. Depending on carrier used from LAX to Europe or South America, SEA-LAX (stopover) - Europe/South America might cost the same number of AAdvantage miles as LAX-Europe/South America by itself.