The Patriots of the '02 Super Bowl. Easily. I remember I lost a bet with a friend who had the cajones to roll the dice on the Pats for $300. The Rams really weren't the same after that Super Bowl. From '99-'01, they sported an offense that most coaches, reporters, and commentators touted as unstoppable, "The Greatest Show on Turf", where they had co-MVP-type players in Kurt Warner and Marshall Faulk leading the way. Stupid Rams defense. I reckon they were taking it easy on the Pats' drive that led to the Vinatieri game-winning FG, thinking the Pats would be conservative enough to take the game to OT. In relative comparison with the Patriots of '01, the Cards most definitely have a shot to take the Series.
But if we're taking into account of all the upsets in history, the king of all upsets had to be the 1980 Miracle on Ice. As a Loyola Marymount grad, I have to believe that the 1989 Elite Eight run, on the same year that Hank Gathers past away in, if LMU got past UNLV to get into the Final Four, take the last two games to capture the National title, that would've been the greatest feel-good story and greatest Cinderella story in the history of sports.