Quickie Trivia Answer: Tampa Bay. I didn't give credit to those who guessed too many teams.
The Nationals have hosted every team in the National League this year, save Cincinnati, St. Louis, and Frisco, as well as two of the three AL expansion teams (Seattle and Toronto) since 1971. And of course, before moving to Arlington, the Senators played the remaining eleven American League teams. The catch: Washington played in the National League for a few years in the 19th Century; the Reds, Cards and Giants all previously visited D.C. the last time the city had a team in the Senior Circuit, in 1899.