Hitler by Shigeru Mizuki
I was browsing a manga section today and just bought this on a whim because it looked interesting. This was published in 1971, so it may be the oldest manga I've ever read to completion, but I'm probably forgetting some Nagai or Tezuka one shots.
As you can probably tell. the manga is about Adolf Hitler. It isn't goofy like Mudazumo Naki Kaikaku, although with Mizuki's characters being very cartoony you probably couldn't tell, in stark contrast to his realistic and traced interiors and objects. Serious people like Churchill, Goebbels, Eva Braun and Hirohito are portrayed almost as caricatures, but without the whimsy (for the most part.) I wasn't aware Mizuki had lost his left arm serving in the Imperial Army during WW2 which make me respect him a lot more as an artist. It's more biographical, focusing on Hitler the man from his artistic beginnings in Austria to his unceremonious death at the end of the Second World War. Mizuki being Japanese and serving in the Imperial Army has a pretty interesting viewpoint from a Western perspective. Instead of focusing on the War (which is not the focus, but obviously is a major part) or the Holocaust, or the Americans or even the Japanese, this is solely focused on the Man himself. Perhaps it is that Japanese viewpoint that allows Mizuki to avoid the temptation to delve into the matters of the Holocaust (which is hardly mentioned at all) and instead choose to portray Hitler as a human and not a soulless monster, which adds a rare level of dimension to the work.
It is a rather odd manga, very conservative and stylistic paneling, but for an older series by a veteran mangaka it serves its purpose as a biography well.