First of all, we are probably all biased. I know I am - I'm going to recommend that you pick either AA or UA.

I fly in & out of Seattle every 2 weeks. I use UA for the simple reason that I connect through DEN, which I like better than the AA option (ORD). Now that I'm hooked into UA, I'm addicted to E+. And I have yet to have a desired upgrade denied as a 1P. (Yes, I realize I'm not trying to upgrade the most desirable routes in the system, but still, a 1024-mile segment is a pretty good spot for 2 e-500's.)

The AA advantage is - I think - some nonstops to New York City. I don't believe that UA can get you that. They'll get you to IAD and CLT (on US metal) - that's your East Coast selection. Plan B is one-stop through DEN, which is better than ORD in the winter. Another AA advantage is that Golds can prebook exit row. On UA, you need 1P to do that.

Anyway, I don't know much about CO, other than the fact that FT'ers hate their mileage program. They seem to have a unique international route map with some cool offerings that others don't have, as well as an international premium cabin product that is well-regarded. I just don't know how hard is it to upgrade into without buying the revenue tickets.