Yglesias nominates for "Worst Intersection in DC" this one, at the confluence of RI, FL, NJ avenues, S street and 4th street NW. Its funny since I used to live two blocks away from this and it still features prominently in my bike route to work. The tangle is exacerbated by DC's overly-restrictive traffic laws - for example there are no guarantees that you'll be able to (legally) turn the direction you want to at any given light.
In fact, it's better not to drive anywhere new in DC without having google-mapped it first. Between the diagonal streets, the circles, the one-ways, the bridges, the Mall, the Park and other distortions, it's not a very fault tolerant city for driving.
It's not so bad on a bike, but could stand to have a lot more bike lanes.