Super Bowl 42 easily. The only other loss that comes remotely close is the other Giants loss in Super Bowl 46 because that Giants team was 9-7 and came within a game of not even making the playoffs that year. But because we’ve seen what the Giants are capable of (and the fact they had already come into Foxboro and beat us that season) it wasn’t as shocking to me.
And if you want to exclude any Giants-related game, the playoff loss to the Jets in Foxboro was a shocker to me. That 2010 team was on fire for two months straight headed into the playoffs and then just completely face-planted.