Monster, Ajin Recommendations
I consider these two anime the two best Thriller anime i've ever seen. And debating, Monster is probably the best Thriller anime existing.
They both have really smart MCs and deliver a story, which seems unbelievable hard for them. You could not think of a way for these characters, to solve their problems and the surrounding crimes. Both anime are dark, gloomy and real. At least they feel so.
Both anime have several plot twists, interesting characters and story lines and deep deep character development.
Dark, psychological, realistic animation, intense, lots of action.
Great character and story development.
Morals of society of what's right and wrong/ good and bad.
Some good plot twists showing us what we really didn't expected. We might hate some characters until we see their reasons of acting one way or another, and then you see it all differently.