The fact that all three DC metro proposals made the cut leads me to believe that one of those three (and possible more than one of them) will be the final pick. Bezos bought a house in DC for $23 million and now owns the Post, and each of the three DC metro area finalists seems to fit most of the criteria Amazon set out in the RFP (although good luck getting from anywhere in Montgomery County to an airport in 45 minutes during daylight hours).
I'm inclined to agree for the reasons mentioned above, plus the fact that employees could live anywhere in the region and still be reasonably close to work, regardless of where the actual facility winds up being built. There are three major commercial airports which serve the area: Reagan National, in Alexandria, VA, just across the river from DC; Dulles International Airport, about an hour's drive from DC; and in Maryland, Baltimore-Washington International Airport (BWI), which is just outside of Baltimore (just over an hour's drive from DC). There are also a couple of smaller regional airports. There is public transportation, including a subway system and bus lines within the city and the close-in suburbs, with also a train system (VRE) which goes further south into the further-out suburbs. There are major highways connecting the suburbs to each other and to DC. There is a large percentage of the workforce which is highly educated, as well as a need for increased employment opportunities for everyone in the region, and of course also the federal government is right there in DC as well (which could provide access for lobbying). Several universities and also community colleges in the area, so educational opportunities are available. Cultural activities available, too, both in the city itself and in the suburbs. In both Montgomery County (MD) and Northern Virginia (Fairfax, Prince William, Loudoun Counties), especially in the latter two counties, there are areas which have not yet been built up so that there is land available for developing and constructing whatever kind of campus they would choose.
All that said, things are not quite as rosy and glowing as might be first thought from what I've written above. Traffic conditions are increasingly congested and there are significant problems with the Metro subway system. Housing is expensive in the region, which could be an issue for some employees looking to move here from the current Seattle headquarters. Weather conditions: pleasant autumns and springtimes, sometimes mild winters, sometimes snowy and cold ones; summers are usually pretty hot and very humid.
It's going to be really interesting to see just what location Amazon does choose in the end!