I may be the only one to feel like this, but I think a severely underrated loss was the 2007 AFC Championship Game in Indianapolis. The Patriots squandered a huge opportunity, p---ing away a gigantic lead to Manning and the Colts. If they won that game, I have no doubt that we'd be talking about a 4 time Super Bowl winner as opposed to a 3 time winner.
That loss hurt for a long time, and I still can't really think about it without getting aggravated. The Super Bowl loss isn't as bad to me, simply because the Giants outplayed the Patriots completely for most of the game. Although I still despise Tyree.