Let's see... I reckon you might've seen more than me, but going by what I've watched that's tagged horror... Highschool of the dead is, despite being very ecchi, really good zombie apocalypse that you don't really see in anime. Gakkou Gurashi is also a nice zompocalypse. Parasyte and Tokyo Ghoul are also listed. Parasyte was amazing but was put off by the censoring in Tokyo Ghoul.