This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / The Completionist
In their New Super Mario Bros. U review they met a baby Yoshi which they named "Fat Baby Yoshi" which they then Squee! over when they see he sings throughout the episode. Then Jirard accidentally dies while holding Fat Baby Yoshi.
Greg's departure from the show. It was bad enough that he was leaving, but hearing afterwards that the two are no longer friends made it much worse.
Jirard's farewell video. Jirard himself seems visibly upset throughout the video.
Jirard: This is one of the hardest videos I've ever had to make... in the history, I think, of my whole life.
What makes it even worse is that when Jirard responded that they were still "friends to an extent", Greg replied that they were not, and then later tweeted that Jirard was lying and he did not leave the show willingly. He later deleted the tweets, possibly due to legal claims about an NDA. Jirard's response to Greg tweeting that they weren't friends is heartbreaking.
Jirard: Due to Greg's response on Twitter, I feel like I've tried my best to handle this as diplomatically as I can. Everyone wanted to know if we were still friends, and I tried my best to defuse the situation as I can. I'm still not going to disclose what happened. But what I will say is that yeah, we are not friends anymore. It wasn't over a woman, as we both have girlfriend/wives. It was over business and the blurriness of friendship. I apologize if it seemed like I lied, that wasn't my intention. I just didn't want to hurt fans anymore than they already were. And to a point, I'm actually more hurt because of his response. All I ask is for your guys' forgiveness moving forward. And if not, well, hey, I tried. No one wants to see mom and dad fight. I'm so sorry
The departure was also hard on Greg as well. He mentions in a reply to a Reddit post how his friendship with Jirard had been drifting apart due to things like distance and interests (It also doesn't help that he states he wanted to focus more on his wife, which conflicted things even more with Jirard). He seems rather regretful and laments over the situation.
Greg: Here's the absolute truth. I miss the kid I met in the history class I subsequently failed because we decided video games and Subway sandwiches were more important than class. I miss the Jirard that I made laugh hysterically over something that might utterly confuse a random bystander. I miss the Jirard that I built a business and creative legacy with. I especially miss the Jirard who stood next to the alter where I exchanged wedding vows with my soon-to-be wife. Above all, I miss my friend.