I just finished watching Shin Sekai Yori and was absolutely floored by the intensity of the writing and the complexity of its ethical core. Hot off its tail I'm on the hunt for something else, anime or manga, that has the same kind of moody atmosphere and moral ambiguity.

Some other things I would consider similar that I've enjoyed:
Gunslinger Girl
Narutaru
Bokurano
Akumetsu
Puella Magi Madoka Magica
Casshern Sins
Alien 9
Blame!

Any suggestions will be greatly appreciated!

51 Ways to Save Her

in need of romance?
uncommon, sad, sweet

Kokou no Hito

Hanshin - oneshot

Hotel - oneshot

The Last Question - oneshot

Watashitachi no Shiawase na Jikan

Great suggestions, thank you! I've read Hotel and Hanshin already and loved both, but the rest are new to me. I'll definitely check them out.

So you're into torturing yourself, I see?

Goth
Lament of the Lamb
Arachnid

Dystopian future? Battle Angel Alita and sequel

Fetish and other works by the same mangaka.