Inio Asano
The author of Oyasumi Punpun, Solanin, What A Wonderful World, City of Light, Nijigahara Holograph, Sekai no Owari to Yoake Mae, Umibe no Onna no Ko, and Ctrl+T.
Fairly unknown in the manga world outside of Japan, not really famous inside either, but if you start searching for him, you'll find that most people will recommend his works. Solanin is probably the most recognized one, being nominated for the Eisner Award, but his other works are just as good if not better.
His works are mostly slice of life stories (except Nijigahara Holograph), aiming towards stories of young adults growing up, realizing their places in the big world, facing with lost dreams and fracturing relationships, resisting the influence of the adult world on them... or exploring what it means to live in the current urbanized, fast-paced and sometimes oppressive modern world. Young, untalented artists (manga artist or rock bands) are frequently the protagonists of the stories, or people who came to Tokyo from the countryside. Though melancholic and sometimes downright depressing, the stories mostly end with a happy and hopeful note (aside from perhaps Oyasumi Punpun, his only long work, though the story hasn't ended so...), so his works are really leaning (leaning, mind you) towards the idealistic end of the Sliding Scale of Idealism Versus Cynicism, and depicting the world as A World Half Full rather than half empty. Oyasumi Punpun is the only story that spans more than 2 volumes (currently 8 running), and hence, the most cynical of his stories yet.
The best thing about Asano's work is its realism, and surrealism. Some of his characters' design are highly unrealistic, giving a jarring feeling that is unique among Slice Of Life stories. Characters sometimes do strange poses and act weirdly, and elements of supernatural, UFO are common, producing a lot of Mind Screw that is often gone unexplained. These elements however are never intrusive, and are often there in order to further highlight the realism of other things, or to signify that life can be strange sometimes. Whatever the reason, the surrealism touch is probably what makes Asano's work stands out from the rest.
Another celebrated feature among Asano's works is the scenery artwork. Asano uses Silent Scenery Panel very, very frequently, and his scenery artwork is extremely detailed and realistic, with amazing lighting, allowing him to make the simple cramped urban setting become Scenery Porn, highlighting his message that the world can be beautiful if you look at it from another perspective. His character design, aside from the frequent Non-Standard Character Design, is also fairly realistic and detailed as well, probably not the most realistic, but quite close to the most realistic thing you can do with character design while still being faithfully a manga.