Per Yahoo Sports:
(They rank Yang over Woods as #1 all-time).
Jack Fleck took down Ben Hogan in the 1955 U.S. Open. Hogan was seeking his fifth Open title; nobody outside of the Fleck family knew who Jack was. And until Sunday, it was the greatest upset in golf history.
• The unheralded Larry Mize upset Greg Norman and Seve Ballesteros in the 1987 Masters, highlighted by an astonishing fairway chip-in on the final playoff hole. Mize won only four tournaments in his entire PGA career, but he'll be remembered for that one green jacket.
• Hillary Lunke has exactly one win on the LPGA Tour: a U.S. Women's Open win in 2003 over Annika Sorenstam, then the No. 1 player in the world. Not only is it her only win in her career, it's the only top-10 she's ever carded.
• Francis Ouimet, a caddy, won the 1913 U.S. Open in Brookline, Mass. at his own course, winning in an 18-hole playoff over Harry Vardon and Ted Ray. It was the ultimate underdog story, like "Rocky" if Rocky had actually won.
• John Daly, the eighth alternate, came from nowhere and won the 1991 PGA Championship, forever altering both his life and the game of golf.
I think you have to eliminate Daly. Sure he was an unknown, but he was young and had great talent and actually won another major later on. And who exactly did he "upset"? Curtis Strange?
Any upset that occurs in 1913 on a guy's "own course" is inherently suspect to me. This was back in the day when you could "stymie" a guy by hitting your ball between his ball and the hole. Sorry. Similarly, Mize was playing on his home course and against one of the game's all-time chokers in Greg Norman. Seve was great, but not in 1987, when he won only one tourney the entire year.
So we have Fleck, Lunke and Yang. Fleck did manage other wins in his career (including a senior major in 1979!). So has Yang.
I have to say that the greatest golf upset of all-time is Hilary Lunke over Annika Sorenstam. No other wins, no other top 10s. She beat one of the top 3 women's golfers of all-time. Lunke's second best ever finish in a major? Tied for 37th. She has one major that she has never even played in.
Y.E. Yang over Tiger -- huge upset, but maybe #2 or #3 of all-time.