My favourite manga of all time would probably have to be Maus by Art Spiegelman. When I was younger I didn't enjoy reading and hadn't really read any proper books at the time. I was very interested in WW2 though and figured that Maus would be a nicer read since it had illustrations. Though it's more like a really long comic(graphic novel), I counted it as the first book I read all the way through.
It's the retelling of Art Spiegelman's father's story as a jew during WW2.