[Jiggly]

Elfen Lied, and it's still my favorite one

I LOVE that show. I think the opening theme tune/video of that show may also be why I now like Gustav Klimt too lol.

Yeah the opening is so weird I love it, the first time you see it you really don't know what you're watching ahah (specially when it was the first anime I really watched).
If you've got anything a bit similar to recommend I'm all hears

To be truthful, I've yet to find anything quite like it. There's some other dark shows out there, like Blood+ and Tasogare Otome x Amnesia (Dusk Maiden of Amnesia), but they're all pretty unique.
One of the others I watched around the same time in that horror/psychological sort of genres was Gakkougurashi!,