If you google "greatest upsets in sports" there is no shortage of lists, though most have most of the same entries, which is okay. But there are some perplexing things about these lists:
See the first one listed below. I would rank the Red Sox 2004 comeback win vs. the Yankees much higher than 31, though it ranks higher as a "comeback" than an upset technically. But to list James Braddock over Max Baer at number 36 and the Miracle Mets no higher than number 18 seems outright bizarre.
But there is at least one major upset that is not even on any of the lists I found, and which seems totally forgetten: the 1914 Boston Braves four game sweep - the first sweep in World Series history, over Connie Mack's Philadelphia Athletics, with their $100,000 infield. This was a last to first story. Also neither the Braves or the Twins getting winning the 1991 pennant counts ? Bobby Thompson's home run ? (Mazeroski's is there). No Arnold Palmer ? New York Knicks 1969-70 over the Lakers with West, Baylor and Chamberlain and an injured Willis Reed coming off the bench in game 7 to hold down the greatest scorer ever up to that point ?
Anyway, here are two typical entries.