For me it’s the Higurashi when they cry series
I haven’t really been “scared” by a series per se, but that is immediately where my mind went when I saw this question.
Same. It’s less scary and more unnerving but it’s a great series
I don’t think I can be properly scared by a movie these days but the best “scary” scenes in anime that made me feel uneasy were in Mieruko-chan, even though it’s only a comedy show with some horror mixed in. Some scenes from from the first season of The Promised Neverland may also fit the bill
The last part (or two parts?) of the Vampire Princess Miyu OAV when I first saw it ~30 years ago. Of course, it probably didn’t help that I was watching it at weird o’clock in the morning.
I don’t think any anime was scary in the sense that it made me scream or anything. There are disturbing ones though, one of the most known one being Higurashi: When They Cry.
Oh man, I remember watching that for the first time. It was nighttime and there was a thunderstorm. The exact moment that a character’s fingernails were ripped out there was a massive thunderclap and my power went out. I’ve never before or since screamed in horror.
I don’t watch a lot of horror, but the slow building tension in Shiki got me pretty good when I watched it a decade ago