Tough question for DC. You'd want something:
- No higher than about 400, maybe 450 feet - tall enough to be the 2nd highest landmark in the city, but noticably shorter than the Washington Monument
- Fairly narrow, with a pointed or otherwise embellished top
- That would complement the city's landmark aesthetic - that would add to to the whole without being too much of an attention whore on its own
- That had a whitish color palette, to go with the marble/granite look common to the city
... I'm not sure I can think of anything. New York's Met Life, Boston's Custom House, Chicago's Mather Tower and Philadelphia's City Hall all came to mind, but I think they're all too tall. Seattle's Smith Tower isn't monumental enough, and the blocky office portion is too high. LA's City Hall strikes me as not quite the right shape for DC, although it's about the best domestic option I can think of.
Might have to take something from Europe. The problem w/ most modern buildings is they're too wide.