I don't have a top 10, but I will add the first one to the group.
1- Secretariat winning the Belmont Stakes.
This is tough because there are so many sports, and most of them have more than one defining moment. Some to consider that may not have been listed upthread yet:
Olympic Gymnastics: Nadia’s perfect 10 (1st ever), Mary Lou Retton’s perfect 10 to win US gold, Keri Strug’s vault on a bad leg to secure gold
Olympic Swimming: Spitz’s 7 gold medals, Phillips’ 8
College basketball: NC State buzzer-beater airball (pass?) tip-in win over Houston, Villanova beating Georgetown
World Cup Soccer: Maradonna’s “hand of God” goal
Professional Golf: Tiger Woods destroying the competition by 10+ strokes at Augusta (Masters) and Pebble Beach (US Open)
Olympic Track: Breaking the 4-minute mile
Bobby Orr: The Goal.