Matt Serra is retired and now scouting future UFC talent on "Lookin' For a Fight", but his biggest achievement was capturing the UFC welterweight title with a stunning knockout of Georges St-Pierre back in 2007.
I've seen Miesha Tate over Holly Holm and Nate Diaz over Conor McGregor called"stunning upsets", but those results really weren't that stunning. The way those respective fights ended may have been stunning turnarounds within competitively matched fights, but the actual outcomes weren't shocking.
Matt Serra knocking out Georges St-Pierre at UFC 69 (on April 7th, 2007)?