I've gone on and on about the National Portrait Gallery before, but I wanted to recommend a couple shows up right now after seeing them myself.
Kehinde Wiley's pieces in as part of the RECOGNIZE exhibit that Fiona talked about earlier this year...I mean, I mostly love Ice-T because of SVU but this is on a whole other level:
watch out though...you're not allowed to photograph this exhibit even though I did.
Ballyhoo!--dumb name, but awesome poster examples of portraiture. The posters really capture design aesthetics of their time and are truly beautiful:
Lastly, the Katherine Hepburn exhibit is tiny but cute--they even have her famous red sweater!
P.S. Aren't the online exhibit pages amazing? You don't even have to go to the museum! But you should ;)