In response to Muzwell's comment:
I think we confuse great moments with great plays. Some of the "great plays" mentioned weren't all that extraordinary. Home runs are really no big deal, they happen every game for the most part. To me the Vinatieri Snow Bowl kick was notable because it was in a pivotal moment in a huge game, but also because it was an extraordinary feat. Same with Tiger's chip in at Augusta.
On the other hand, Bobby Thompson, Fisk, big moments but not anything remarkable as athletic achievements.
I'd add Kerri Strug's Olympic vault with a broken ankle, huge moment, extraordinary achievement.
Not one play, but Reggie Miller scoring 8 points in 8 seconds to beat the Knicks in the playoffs, Buster Douglas KO of Mike Tyson, Hagler/Hearns, Ali/Foreman.
To me, producing in the clutch advances a homerun to "great play" status. Kirk Gibson hitting his homerun on one leg made his homer special, and turned that series around. Carlton Fisk winning arguably the greatest game in baseball history.
But, Reggie Miller's 8 points in 8 seconds wasn't "a play". It instead qualifies as a feat...like when Wilt Chamberlain scored 100 points in a game...or when Reggie Jackson hit three homers to clinch the series against the Dodgers in "78...or when Ali did what was thought to be impossible, in knocking out Sonny Liston, and, later, George Foreman.