On an emotional level, you're going to get different answers on what Packers loss has been the most devastating. For me, the most viscerally devastating loss was the '98 Wild Card--Young's bomb to T.O. with just seconds left, after Favre's TD to Freeman w/1:56 to go made it look like the Packers would win on the road.
Historically, however, this is worse, because it didn't come down to one play, like a Rodgers fumble or Favre interception. It wasn't a competitive game, and too many players made mistakes. You can't pin it on one player or one play. For a 15-1 team to get rolled at home like this, it's perhaps the worst playoff loss in team history.
I do think they'll learn from this, and there should be no reason at this point to think that they won't be SB contenders next season.