Much depends on price range, what they like for amenities, and where they want to be in the city. Also, when they want to go...prices in DC right now are through the roof...I had to pay $365 last night for a room that I paid $225 for in August!
Here are my votes in various categories:
Boutique Hotels: The Monaco and the Sofitel - both downtown, very hip, funky fun. The Sofitel is French and has a real European flair. Topaz - closer to Dupont Circle, same brand as the Monaco, very nice.
Luxury Hotels: Hay Adams, St. Regis or Willard - all downtown, near the White House. Very very elegant. Can sometimes get amazing rates on weekends since they tend to be business hotels during the week. We once paid $150 for the St. Regis and got upgraded to a suite!
"Chain" Hotels, non-budget: Westin Grand, nearer to GW and Georgetown, in a lovely neighborhood. Omni Shoreham, up near the zoo also in a very nice neighborhood, near Rock Creek Park, more residential. Capital Hilton.
"Chain" Hotels, budget: Sheraton Four Points on K Street, the neighborhood is much improved there and the hotel is nicer than you'd think. Hilton Garden Inn and Homewood Suites downtown are ok. Embassy Suites on 22nd is a good choice in a good neighborhood.
Hope this helps!