I’ve either arrived or left from all of IAD (Washington Dulles), DCA (Ronald Reagan) and BWI (Baltimore-Washington International) now so I think I’d be a reasonable judge to suggest which airports to use if you do want to go there.
Personally, whilst IAD is probably the biggest and most used of the three, it’s also the most inconvenient in terms of finding your way to the main part of the city. Whilst they do have a bus to take you to the Metro, it’s also overpriced compared to the alternatives at the other airports. And forget about taking a taxi – the average price of a taxi from the airport to the city is about US$70.
DCA is the most convenient – it’s right on the Metro, and the closest airport to the city. So if you’re wanting to visit Washington, this would be your best bet. However, BWI is also a good alternative if flights end up being cheaper or more direct. You can get a free shuttle to the nearest Amtrak station and catch an Amtrak or MARC (Maryland commuter) train directly to Washington’s Union station, which connects to the Metro, and only takes between 30-40 minutes.
BWI is quite a nice airport, too, and includes an observation level to watch planes, a small museum area about different airplane parts, and a playground for little ones. If you’re stuck in the airport for a full day like we were, this is certainly a nice one to be in and I was grateful for the playground. It was so popular in fact that locals not even flying bring their kids there.