We're obviously all C's fans here, so I assume a lot of answers will involve the C's, but feel free to include other teams/sports as well. Which loss (game or series) has been the most difficult for you to get over?
For me, it's hands down Game 7 of the 2010 Finals. Unbelievably gut-wrenching and depressing, and the way we got jobbed by the refs in the fourth quarter makes it much worse. That team just seemed to have "it" and be able to find a way reach another gear when it needed to, so there wasn't a shred of doubt in my mind about us winning that game. That loss still hurts - a lot - for me to think about.
I'll throw in an honorable mention, and it's the Warriors losing to the Cavs last year. I'm not a Warriors fan at all, but absolutely despise the Cavs. I lived in Northeast Ohio for about a decade, and the vast majority of Cavs fans are pure bandwagoners who couldn't name more than two players that have ever played for the Cavs when LeBron wasn't on the team, either pre-2003 or during his time in Miami. And the way they immediately welcomed LeBron back with open arms baffles me, and I felt the NBA did everything it could to give the Cavs a chance in that series by suspending Draymond Green for Game 5. In all sports at all levels, the Cavs were the one team I never wanted to see win a title, especially with LeBron on the team, so that one made me sick.