Alaska. In my limited experience (connecting from AA) and from everything I've read, they're a great airline with a great frequent flyer program. They have upgrade reciprocity with Northwest and will soon with Delta. (Alaska might not work for you if you fly ST airlines that aren't Alaska partners, though.) Alaska's partnership with Continental is much more limited, so they aren't worth as much here.

I'm in a similar boat, except I don't have SkyTeam or Star Alliance elite status to worry about; I plan to open an AS account for DL/NW flights once Continental joins Star.