I nominate
Doubutsu no Kuni (Animal Land) by
Makoto Raiku
Doubutsu no Kuni (Animal Land) is one of my favorite manga by one of my favorite mangaka,
Makoto Raiku. Mostly known for Konjiki no Gash Bell!!
(Zatch Bell!), which is one of the most underrated shounens (especially in the west) there is. Doubutsu shares a similar fate, only worse, by not just being underrated, but also overlooked.
It's an excellent Shounen manga that tests the boundaries of it's demographic more than once, while the style might not immediately show it, it's filled with darkness and despair throughout.