As a participant, I'd say it was a tie between the first time that I red-pointed Ring of Fire (
http://www.mountainproject.com/v/ring-of-fire/105789153) and the time that my soccer team, with me playing sweeper-stopper, beat the Seattle Sounders 1-0 (I think it was 1987 and the Sounders were not the team that they are today, by a long shot).