This might not be the most important game, or it could've been. But just the game that took you to that next level. First ever victory? first ever victory with a horrible team? first ranked victory? National Champ victory? first NT victory? Shoot...it might even be a loss.
Mine was against then #9 Redland (a current rival), can't remember the season, at least 10 seasons ago. I was unranked, with possible RPI of a billion. First game of the season. I beat Redland in a convincing way, dominate. First win over a top ten team. I knew then that finally I could play with the big boys.