Seconded Eighteenth Street Lounge.  

I also recommend the new St. Yves in DuPont Circle, an absolutely beautiful establishment. Since it is new, the menu is still quite limited (but they have an affordable caviar service)

Depending on the kind of set up you like:
Cloak and Dagger or Denson’s for speakeasies
The Graham or POV at the W for rooftops
Marvin or Barcöde for a lively crowd  
The Dignitary (in the Marriott Marquis), Jack Rose, or Shelly’s Cigar Bar for eyeing handsome men