I love DC most in the winter. It's stunning in the spring and summer, sakura wrapping around the Tidal Basin, and festivals giving the city a bubbling exuberance. Fall is a beauty to behold as well, but wintertime just always felt right.
Because I can't just leave well enough alone, here are a few token federal buildings for your viewing pleasure:
Supreme Court at sunset
Library of Congress, Jefferson Building--a shout out to my librarian friends!