When Romeo died in Romeo x Juliet. It was just so sad and touching.
(And I don't think that needed a spoiler, btw.)
I was actually irritated at his death in the anime.
Spoiler for RxJ:
I thought it was a really selfish and idiotic death. He got himself killed because he was trying to stop Juliet from sacrificing herself to save their world. I guess some people can see that motivation as romantic, but I think dooming your entire world to save your love(who's just going to die with you when the world collapses anyway) is not really a smart thing to do.
Riku's death was pretty much the turning point of the series. Having your new step-brother raped and killed shortly after getting your blood sucked at a dangerous level is enough to pretty much change your whole world. After having your adopted father killed. >;