Me it was when Baltimore played Calgary in the Grey Cup, Canada vs USA. Of course, I was cheering for Calgary and was very upset they lost, but Baltimore would eventually become the Montreal Alouettes the following season, and I started to follow. They also became the Baltimore Ravens, but who cares about them 8)
But that was the game that got me into football, and it is ironic that the following year, my city would geta team, which would become my favourite since the first game I saw them play.