Darker Than Black Blue Gender Elfen Lied Mirai Niki Another Berserk Claymore Akame Ga Kill Blame (honorable mention) Knights of Sidonia, more of a sci-fi anime, but I think it's dark, nonetheless.
That is a good choice. Technolyze and Casshern Sins are probably top contenders as well. And of course Ninja Slayer From Animation because that flash style story is quite dark. Most of the low budget anime stories tend to be dark to save budget. Betterman has a very dark universe except for the silly title character itself who goes around fighting the evil monsters. There’s also a good argument to be had that Afro Samurai has a very dark outlook on life especially with all the crazed fighting and endless struggle to be number one or number two. I mean, people keep dying in that universe. Tons of Isekai are dark but mainly that’s due to the ease of copy pasting dark revenge stories for easy profit.
Happy Sugar Life - (This anime contained every twisted dark matter as mentioned below) Spoiler: Summary pedophilia domestic abuse murder Twisted concepts of “love” Monster (darkest I have seen in a while) Puella Magi Madoka Magica
Darker than Black Deadman Wonderland PsychoPass Another Akame ga kill (spoiler: they kill for no reason) Gleipnir Tokyo Ghoul (manga has much more depth and is wayyy darker)
death note. saga of tanya the evil. In anime, i have only seen these 2 which suited my taste and i can say they are dark. anime like elfen lied i just removed after 1st ep idk why lol. its been a while since i watch such a anime. Its very rare, anyway it just depend on taste ig