I just had one last night playing through P3: FES. This evoked a response I didn't think I would get but I would like to avoid too many spoilers.
Doing the final "date" request with Elizabeth to visit the human world and choosing "Boldly Accept". I did not think I would get that outcome cause most of the Elizabeth requests are generally light hearted and comical. It really changed my perspective of things and oddly made me come to be more attached to Elizabeth. I know this is vague but those who played FES will understand.
I would also like to add Akihiko and Junpei getting their Persona upgrades as powerful and emotional moments.