In winter the wind generally blows from the north, so you want to be on the DEF side of a 737 when departing DCA. Arriving at DCA you will not see the city, but could have nice views of Alexandria from the ABC side of a 737.

If the winds are from the south, you will have an excellent view of the city while landing if you sit on the ABC side of a 737. If you are on the DEF side you'll see the Pentagon and Arlington Cemetary. Taking off with southerly winds you don't see much of anything.

Note there is nothing to see whatsoever when taking off from or landing at IAD so if your question relates to a LHR-IAD run you're out of luck. Sorry - Dulles Airport is 30 miles from Washington.