Probably the same scene tbh due to the permanent changes it meant for the series. That or a select few of the character deaths in LotGH due to how well written it's cast was.
If I had to pick another one, perhaps Meruem and Komugi's death in the anime due to the build up towards it and the good directing behind it with both the painting-esque art and the music prior to it making it a wonderfully crafted scene. It also allowed me to have mixed thoughts on it, as despite feeling that the two had only started their journey together, it felt like a necessary end to their character arcs. If including manga, then some of scenes from Real are contenders as well due to how the characters' situations really allow you to feel a lot of empathy towards most of them. Not many other series I can think of, at least not ones that'd make me sad at all now, other than Part 4 JoJo's ending purely because where's the Part 5 anime?!