Depends where you're going in WAS. If you'll be working downtown, DCA is very convenient. You won't need to rent a car, as the airport and most hotels are on the (very good) metro system. But you'll probably pay a slight premium in ticket prices.

If you'll be working in the VA suburbs and will likely rent a car, IAD is a UA hub and has lots of other options as well.

BWI is the cheapest, but a real pain to get back and forth from unless you're working in the MD suburbs north of DC.

You might pick an airport, then use Expedia or Travelocity to run some of the dates you'll be traveling and pick your carrier based on prices and schedule options.