Denver still makes the most sense to me. Single hop to Beijing & Shanghai, real estate and wages aren't at DC, Seattle, Bay Area rates quite. Lots of amenities (open space, good beer, close to skiing) that Amazon doesn't actually have to pay for. Decent tech scene (Google, Facebook, Uber campuses, as well as an ever growing Amazon dev presence). Large downtown property looking for a buyer and is probably underrated (Elitch Gardens). Decent public transit that's trending really well (new light rail within the past couple of years, and more coming out soon).
DC never made sense to me. The other defense contractors manage to not have HQs in DC.
I think this story is exactly what they'd leak if they were trying to buy real estate somewhere else.