Attack on titan is good. And I totally encourage you to look into the manga once you are finished with the current material.
So let's see.
If you want horror, Parasyte was decent. I prefer the manga, but the anime should be fine. Shiki is a nice, somewhat underrated horror anime as well. Idk. Anime is not what I go to for horror.
For action: Fullmetal Alchemist: Brotherhood, if you have not already watched it. Baccano. Black Lagoon. Jojo's Bizarre adventure (can also technically qualify as horror). Escaflowne.
Dark: Knights of Cydonia is not that bad to dovetail into from Attack on Titan. Neon Genesis Evangelion, though I guess bleak would be the proper term. To watch anyway eventually. Berserk - manga is a must eventually. Monster. Welcome to the NHK. Puella Magi Madoka Magica - most of Urobuchi's stuff: Fate/Zero, Psycho-Pass.
Idk. That should give you quite a bit of material to chew on.