haven't seen it listed yet, but my second favorite was the season ending game in 2010, when Justin Maxwell hit a walk-off grand-slam against K-Rod and the Mets.
The Nats were awful that year, the Mets were heading to the playoffs IIRC. Maxwell fouled off the pitch just before the HR and it landed behind us. The fan who caught it gave it to my son.
There wasn't much to cheer for that year, but it was a nice way to end the season.