"The Catch" game (1982 NFC Championship game between the Cowboys and the 49ers).
The very-beatable Bengals were waiting to play the winner in the Super Bowl and I never wanted my team (Cowboys) to win a game more.
It hurts just thinking about it
A close second was the 1995 NFC Championship game (once again Cowboys vs. 49ers). The Cowboys were going for their 3rd consecutive Super Bowl and with the very-beatable Chargers waiting for them.
Troy Aikman opened the game with 3 INTs (which the 49ers converted into TDs and a 21-0 deficit). The Cowboys fought back courageously but came up short 38-28
All that was back in the day when I rooted for a TEAM. None of this "The Cowboys can win but they have to allow Warrick Dunn to run for 120 yds. and a TD and they can't score more than 20 points on the Atlanta defense...".