Not listing 2nd seasons, and only pulling from what I personally have watched. These are shows I consider to be the best "dark" anime. Alphabetical order.
I love the music in anime and the opening can often be a deciding factor on whether I should watch the series or not. This also includes series I didn't particularly enjoy but the music was great...
This list is rather self explanatory. It is however, worth mentioning, that I will only be adding the first season of each anime series onto this list.
Challenge by Larkawolfgirl
But don't take my word for it, opinions and tastes differs from everyone. But this is a list where I add animes I would recommend to my friends.
My favourite, top rated anime.