Some of the big matches that will get votes here may have been pre-contest upsets, but by the time the match was set, the result was expected. So I'm voting for matches where the result was stunning on the match day. I'm making a mild exception for the last one, because Kuja lost to a fucking glove.
Knuckles > Magus
Squall > Luigi
Pac-Man > Revolver Ocelot
Master Hand > Kuja
Azuarc
is my favorite arc of the Game of the Decade 2020 anime.