Which DC airport to fly into?

Will be staying at the Hilton Washington...flights cost the same so I am trying to figure out which one will be more convient to get to the hotel...

On order of transportation ease:

National
BWI
Dulles

I have stayed at the Washington Hilton. If you are not renting a car, the best bet is to fly into DCA (Reagan National) and take the subway. It stops about two blocks away and is an easy walk.

Reagan National.

However, you should be able to find a couple cheaper flights into Dulles or BWI.

DCA by far----sit on the left side of the plane so that you can see the lights of DC on final approach. All of the monuments are visible, as well as the Capital. Far and away a great night time landing airport. The wind normally blows up the Potomac, so you'll be landing to the south. No view on right side of plane to speak of, except the darness of Arlington National

Reagan National Taxi, Subway or Shuttle bus will get you there in minutes.
I believe the shuttle for the Hilton Washington is Complementary (Check it in advance)

Fly into DCA.

If you fly into Dulles(IAD) or BWI, you'll spend much longer getting from the airport to the city. Not to mention that the shuttles/transportation/taxis from IAD and BWI are more.

All else being equal, I'd fly into National as well. The Metro stops right by the airport. You can take the Blue Line to Metro Center or the Yellow Line to Gallery Place, transfer to the Red Line in the direction of Shady Grove and get off at Dupont Circle. Try to be at the back of the Red Line train because you want the Q Street exit. Turn right coming out of the escalators, going towards Connecticut Avenue, turn left on Connecticut, and walk about four blocks.

WIth BWI, there's a shuttle from the airport to the Greenbelt station on the Green Line. It runs every 40 minutes. The Green Line crosses the Red Line twice, at Fort Totten and at Gallery Place, so it's a tossup as to which is an easier transfer. If you transfer at Fort Totten, you have to walk down two escalators, but Gallery Place is far more crowded.

Dulles is way out in the middle of nowhere. There's a half-hourly shuttle to the West Falls Church station on the Orange Line, and you could take that towards New Carrollton and transfer at Metro Center to the Red Line.

Its always better to fly to DCA, its a smaller airport and closer to DC. BWI has cheeper flights but its far away, I would avoid it. IAD is OK, but security lines can last forever.