I'm just looking at the map, so I am not taking other factors into consideration, but CBus has a decent shot.  I would think Columbus, Indianapolis, Nashville, and Atlanta would have to be tops in terms of being opposite of Seattle and being centrally located.  Some of those cities in the NEast would have a traffic jam from the start--that may be true for Atlanta also. 

I know a lot of trucking companies like Columbus b/c of its central location

https://www.yahoo.com/finance/news/amazon-reveals-top-20-city-141341885....