The best and the least expensive are not the same. The best would be a car/limo straight to your hotel. The least expensive involves a bus or two.
Next time you plan a trip to Washington DC plan on flying to Reagan National. National has its' own METRO stop.
The only time I flew to Dulles I rented a car there for a trip to Columbia MD without driving into the city.

If you've only got carry on's, I suggest taking the Washington Flyer Coach from Dulles to the West Falls Church metro station. It leaves every 30 minutes. You can pick up the Orange Line on the metro and take it to Foggy Bottom. Your hotel is an 8-10 minute walk. Should be $10 for the bus and $3.00 and change for the metro.

There is a lot of track work being done on the Orange Line so check metro's website (wmata.com) to see what's up.

Alternative: Book on the Super Shuttle van which goes to multiple hotels (same company as the Washington Flyer) That runs $30-ish for one with a substantial discount for additional passengers.

A taxi will run you $60-ish.

I took the Super Shuttle last summer with my two teenagers plus 3 bags. We reserved on line ahead of time, met them right at the curb from the baggage claim and they dropped us at the hotel door after dropping off 2 other stops. We were at a hotel not far from the Capitol. Easy, convenient and not outrageously expensive.

Thanks to both of you.

tom, we're flying on Porter Airlines because we love it and they're just starting their Washington flight (we've been waiting!). It just happens to fly into Dulles. It leaves from a downtown airport in Toronto. They're also having a 50% sale. All good reasons.

obx, the Super Shuttle sounds like the best bet. Where do we find it and do we have to book in advance?

Sorry, live, we were posting at the same time. Do I just google Super Shuttle, Washington DC?

Info here: http://www.mwaa.com/dulles/793.htm

Have a great trip!

You're a pet! Thanks!

obxgirl,
Thanks so much for suggesting the Super Shuttle. It was perfect for us. It cost us $20/pp each way (a trip our Washington friends said would cost about $60 or $70 by cab, each way) and it was prompt, clean and efficient. I'd definitely recommend it!

You're more than welcome -- hope you enjoyed your trip to DC!

The best kept secret to traveling from Dulles to the DC mall area (hotels, etc) is the 5A buss. http://www.wmata.com/bus/timetables/dc/05a.pdf You catch it right outside the doors on ground transportation area (clearly marked). Take the bus (about $6 exact amount or smart card) to the Rossyln metro station or the L'Enfant station and get on the metro rail and you are home free. You can figure out the metro station nearest your hotel at http://www.wmata.com/rider_tools/tripplanner/tripplanner_form_solo.cfm Piece of cake.

Thanks, TroyMan. Our Washington friends who fly frequently for work told us about this as well and while the SuperShuttle was more expensive than the bus, it was less expensive than a cab, and just so simple. I don't travel light (eg I'm not a carry-on sort of girl) so the SS was easy-peasy.

tomfuller, I have seen you suggest not flying into Dulles a few times. I am not sure where you fly into DC from but from Detroit and many other cities, the difference in airfare to Dulles or BWI is almost half of what the airfare is to Reagan National or a savings of $200-250.

DCA (National) is almost always the most expensive place to fly into. Its the most convenient to the city though.

In years past, my favorite and usually cheapest airport to fly to was BWI and renting a car or taking Amtrak into DC.
There are car rental desks in Union Station if you need to rent a car in DC.
If you get a great deal flying into Dulles, try a limo to your hotel if you don't rent a car.