This is a tough one to vote for. I decided to pick two.
It was hard not picking "Hey Big Vendor" as it is a great episode.
However, I decided to keep with "Missing Bus of Mrs. Avery" due to the bus trip, Mrs. Avery's eft and rights, and the bus in neutral going downhill.
"Gnome and Away" was my other pick. Billy's dance and lip syncing performance puts this one over the top!