Well, the saddest moment for me is actually two moments from two movies. The movies are Red vs Blue Season 8 & 9 and before anyone says that they aren't movies, they are. They are all feature length and after Season five each season can hold it's own.
So, both of the moments deal with the "main" character and what he says. In season eight he is trapped in a AI unit and is trying to find the one he loves and he says this, "I’d like to say that I found her right away, that I just walked into the Epsilon unit and there she was, waiting for me. As you can probably guess it didn’t happen that way, but I know she’s in here somewhere, and I’ll find her. We always seem to find each other, for better or for worse. I don’t know why the Director did what he did. I don’t know if he was trying to revive a memory from his past or if he was just trying to get it out of his head. But I figured out something that the Director didn’t. It took Alpha, Delta and all the rest to help piece it together for me, but, what I’ve learned is that a great love is a lot like a good memory. When it’s there, and you know it’s there, but it’s just out of your reach, it can be all that you think about. And you can focus on it and try to force it, but the more you do, the more you seem to push it away. But if you’re patient, and you hold still, well maybe, just maybe, it’ll come to you. I just need to make sure I’m somewhere that she can find me. I think this place is a little different than it was before. See out there, everything is based on the Alpha. But in here, well, I guess I'm the Alpha. And maybe this time through, things will be a little different for me as well. I guess I'll find out."
I cry every time I hear that, oh and this "main" character is an AI himself. In season nine he ends up finding who he is looking for, but the AI unit is "failing". So he is talking to her and he mentions how all he ever had to say to her were three simple words. She tells him that if the words are 'I love you' then he might as well stop because she doesn't like good byes. And Church is like 'No, what I had to say was I forget you.' After nine seasons of building up a character, the one who loves her wipes her from his memory. Saddest moment ever.