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J PaulOuiment over Vardon is a great choice but my vote would go for the 1980 Olympic Hockey game over Russia.
Honorable mention - in horse racing the aptly named Upset defeated the immortal Man O'War in the 1919 Sanford Stakes at Saratoga handing the great champion his only career defeat. Some believe that this result was the beginning of the use of the term upset in sports writing for a big underdog winning a sporting event over a seemingly superior opponent.