Being a Washingtonian, I have to lobby for my area. Timing is better if you are at DCA for a layover because the Metro has a platform RIGHT THERE and you can visit Old Town Alexandria (and give me a shout out) just two stops away or sight see in DC just a couple of stops away. There's also several malls a stop or two away and with trains usually coming every 10-15 minutes during the day, you're pretty safe to take a few hours off "campus" during your layover. ORD was a NIGHTMARE for me last time I was there. So busy.

If you end up at IAD, it's not an awful place to be stuck for a few hours but I would almost put it around the same as ORD. So that would be a toss up.

If you're flying into BWI... Do. Not. Leave. The. Airport. MAYHEM. I try to steer clear even though the cost savings are better there.