Packers lost to Broncos in Super Bowl XXXII. 1998. I was 8 years old, but a die hard Favre and Packer lover. I legit remember crying in my room after the game. Probably one of the first sporting events I took really hard.
Two Michigan football losses really standout...the '15 bobbled punt fiasco against MSU. I honestly think that makes that years OSU/UM game a lot closer because of what would have been on the line. Also the 2016 game against OSU....another one that was so close and at that point it seemed like Michigan would never beat OSU.
Next would probably be the 2016 Indians world series. What a disaster that was, but so close to tasting a championship.
Another was probably JJ Redick's last game at Duke when they played like absolute trash and lost to LSU. I was a big time Redick guy for his 4 years at Duke (he played when I was 12-15 years old and I thought I was going to be the next great white basketball player/shooter). That Duke team was loaded that year and I think was the odds on favorite to win it all, although they would've had to go up against Florida in the final.
Fun times thinking back on my misery lol.