By this I don't mean the most heartbreaking loss or the biggest blowout, but instead the least entertaining, completely un-fun game you've ever watched all the way through.
Of course, there are many contenders, but for me this one is forever stuck in my memory as the standard:
https://www.pro-football-reference.c...0410310chi.htm
This was in 2004, when the 49ers and Bears were at the height of their crappiness, and really just a great example of the dullest possible game between two teams who completely sucked at football and couldn't do anything right. A close game, but completely un-exciting as both teams just bumbled their way for a few yards here and there to trade field goals, and for the whole game it was clear neither offense was going to do shit. I was only watching it because I had bet on the under, which was 35.5 points and looking like a good bet. As an added kick in the nuts, the Bears scored a 70-yard defensive touchdown at the end of the game to make it 36 points and cost me $20. I don't know if I've ever seen two more incompetent teams play each other in my life.
The same two teams actually played what was objectively an even worse game the next year, but the Bears were actually a decent team then, so it didn't have quite the same low-stakes undertone:
https://www.pro-football-reference.c...0511130chi.htm
Some of the stat lines though, holy shit: Cody Pickett, 1-for-13 passing for 28 yards (49ers one completed pass all game)! Kevan Barlow, 25 rushes for 63 yards. Kyle Orton, 8-of-13 for 67 yards, a combined total of 95 passing yards for both teams. Eight fumbles, 12 punts. Combined 6-for-29 on third down. 49ers with more yards punting than total offense, by a wide margin.
I sometimes get these two games confused with each other, but I remember the first one being a night game, and that for some reason, it was just the most flat-out un-entertaining thing to watch. It was close, though.