I'm pretty sure UA is the only airline that flies PDX-WAS nonstop (to IAD). Plus between UA/US domestic, and *A int'l, you have pretty good options for going elsewhere from both the west coast and the east coast (and everywhere in between).

If low fares are more important than having a nonstop flight, though, then I'd probably go with NW or CO. Having the ability to choose between CO, NW, DL, and AS (AS also earns EQMs on both CO and NW) lets you take advantage of the fact that they do still compete against each other in the fare realm, while you have far more options for routings and can still earn miles on any of them and even qualify for free domestic upgrades on both CO and NW (and, to some extent, AS). Plus Skyteam gives pretty good coverage internationally (as long as you don't need to go to Australia). The only big drawbacks to NW/CO are (1) the difficulty of upgrading int'l tickets with miles, and (2) the difficulty of using the miles you earn.

In the redemption department, AA probably takes the cake.