I've got a few answers. It's too hard - GameFreak hits with all the feels sometimes.
Firstly, I'd like to agree with the PMD series endings - I played 1 and watched an LP of 2 - I cried so hard on #2's ending, it was insane.
Secondly, I'm an avid nuzlocker. One of my favourite things to do with my spare time. I once competed against a friend of mine in a nuzlocke of Pokemon Black. I wiped against Grant and his Excadrill, despite going in with at least one type advantage against it. I slugged away at that game, trying to catch up with my friend. In the process, I lost my Leavanny which I loved with all of my heart. I couldn't bring myself to release it, so I boxed it instead. I caught up with him only when I finished training everything to level 55-ish for the E4 & etc. I went in, screwed up where 3 of the E4 were, despite a lack of casualties (my friend lost his Swanna to Marshall, though). During the Reshiram fight, I lost my Ferrothorn because I forgot that Reshiram was a fire dragon. My friend and I went in, fought N, and won. At that point, we battled Ghetsis and got our asses handed to us (it was either by him or N, I'm not 100% sure anymore). I lost twice. I then went up to battle Ghetsis again. By some streak of pure luck, Reshiram survived Hydreigon's attack with a sliver of health, giving me just the opening I needed to go in and own Ghetsis' ass off. After being asked to nuzlocke the postgame, I said I couldn't - I wanted my Leavanny back.
Special mentions to the Distortion World in Platinum, N in BW, the memory link in BW2, and the ending song in XY.