Football: 2009 Grey Cup, with the insane "too many men on the field" ending. Being one of only 3 people in my immediate area cheering for Montreal made it even better.
Baseball: Justin Verlander's no-hitter in Toronto. It was 9-0, or some blowout score, at the time, so we were safely at a point where you don't feel guilty cheering against the home squad. And one walk away from perfection.
Hockey: I've never been a Leafs fan, but when a buddy told me his dad had some extra tickets in the grays for a playoff game, I couldn't refuse. Which had me in the building for Gilmour's this-way, that-way, this-way-again goal in double OT vs. St. Louis.
Basketball: I haven't been to many, but seeing the Raptors wipe out a 20-point deficit to beat the Wizards in 2005 was probably the best.