Recommend me some single season Anime?

Deadman Wonderland. The manga is pretty short itself too, but I think that's to its advantage: it sets out to tell a story and tells it, without adding filler just to extend things.

ETA: Sorry, I should tell the story!

It's about a guy who was framed for murder and gets sent to prison, and discovers that the prison is running less-likely-to-kill-you death games for the general public's entertainment on the main level, and then underground, runs even more gruesome death games for rich people to bet on.

That sounds a little Squid Game-y, but it's really not. The MC is interested in unraveling why he was framed, and there's an element of magic/scifi to it all.

Anohana for sure