DCA or BWI
DCA is closest by distance but BWI might be faster during rush hour. I landed at DCA at 4pm, my colleague landed at BWI at the same time; it took me 1h10m to get to the hotel, it took him only 40m.
BWI
BWI is definitely the way to go!
BWI is the easier airport to access by car, National has the metro.
BWI is easier to get to/from CP than DCA and quite often airlines offer less expensive fares to BWI.
BWI is most direct
DCA but you should consider BWI also is good choice
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