Black Lagoon, Rainbow: Nisha Rokubou no Shichinin Recommendations
First off: This also includes the Secound Season of Black Lagoon as well.
Although Black Lagoon and Rainbow don't have much in common concerning the storyline, characters and such, they are incredibly similar in one perspective: The atmosphere that is created while watching the story unfold. For the most part it is the same dark and sometimes heavy atmosphere where hope or other positive things dont exist at first glance and are hard to find. I admit, I lied there is another similarity: The deeper psychological meaing behind those animes, you can find many different ways of interpretating these series in both of them and let
Both shows tries to show you the dark side of the world, the suffering to live in the world of crime, and main characters with distressing backstories.