For me, it was the infamous "The Leafs lost to a 42-year old zamboni driver, who works for them" game against CAR in 2020. I personally see that game as not only the most humiliating loss in Leafs history, NHL history, hockey history, but all of sports history (if anyone has one that's worse, please share it for the sake of my morbid curiosity). It was not only the loss of that game, but the fact that Sheldon Keefe cancelled practice the following day, presumably so that the players wouldn't have to face scrutiny from the media. Lastly, Kyle Dubas claiming that the opposing team losing both of their starting goaltenders and so you get to shoot for half the game on a human shooter-tutor is a "no win" situation.
People like to meme and joke about this game and it's obvious why, however, if you look at it from a serious perspective, that 24 hour span truly shows how incompetent this organization is, top to bottom, from its soy-infused GM, to its cowardly coach, to its soft, gutless players. And sitting at the top of shit mountain is Shanahan, the carnival barker, who always somehow manages to escape criticism but should also be gone for allowing this obvious circus to continue unabated for his entire tenure. This organization is completely lacking in accountability and has been for years. We saw this ugliness rear its head again just recently when Keefe had to walk back his (perfectly valid and 100% correct) criticisms of his pampered prima donna stars. The inmates run the asylum in Toronto and this has been obvious to me for years. I'm glad other people are finally waking up to this fact.
You want change and success in Toronto? This entire organization needs to be gutted, top to bottom. Shanahan needs to go, Dubas needs to go, Keefe needs to go and this core needs to be blown into unrecognizable pieces. Then you start building from the ground up once more. Yes, it's horrendous luck (and just so Toronto) that when they finally draft and develop and hit on so many great picks that they all turn out to be spoiled immature children that worship at the shrine of failure when it comes to team success, but continuing to "run it back" for years and years with the same result is an even bigger L that is entirely self inflicted and completely unnecessary.