Seinen Manga Recommendations?

Now for those who don't know, Seinen manga is meant for a more mature male audience. It is the male equivalent of Josei manga. Seinen titles are typically darker and more somber than their Shonen counterparts.

The only one I've read so far is Bitter Virgin.

If you have any recommendations, please list them here, but don't just say "Read this!!! It's awesome!!", at least tell me the story behind it without ruining the story.

Thanks in advance :D