Despite Tezuka Osamu's fame, I don't think many people have actually read his works. I recently finished rereading Tetsu no Senritsu - the main story and its accompanying oneshots are nice.

Try God of Bath, its deep, philosophical, quirky and funny at the same time. Very underrated gem. Alot of this author's work's are pretty cool, though I liked this series the best.

Gokuaku no Hana - Houkuto no Ken - Jagi Gaiden is the best thing that happened to the franchise since the point of Raoh´s death in the main series but i doubt that many read this prequel.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

Try Yohohama Kaidashi Kidou.

[url]http://www.mangaupdates.com/series.html?id=586

Very few of anything are actually unique, but this curiously gentle post-apocalypse story really is.

Houseki no Kuni
It's so good! So overlooked!

Liar Game. Frequently compared to Death Note, but definitely smarter and criminally underrated. 😃

MONGGO by SLAIUM
A malaysian manga that I think no one will ever know.

The first manga I ever bought is Doraemon!

Quote from zarul

MONGGO by SLAIUM
A malaysian manga that I think no one will ever know.

Well I can't find it so I'm guessing it's not translated.

As for me I'd say How I Stalked Some Dude with an Exposed Nipple and Stumbled Upon the Zenithian Sword is absolutely underestimated. Not underrated since everyone seems to like it, but it's easy to just skip it because of the crappy title.