Anything by Mizuki Shigeru- not just Gegege no Kitaro, but his historical/autobiographical work as well: Onward Toward Our Noble Deaths, Shouwa: A History, and Nonnonbaa are amazing. Tezuka, the 'god of manga' is actually more hit and miss (and a lot of his aren't actually finished). Black Jack, Phoenix(considered by him his life's work, unfinished at his death), Buddha, and Astroboy are all classics, though. Read Astroboy long enough to get to "the Earth's strongest robot" tale, then read Pluto by Naoki Urasawa, a modern retelling of it. While we're speaking of Urasawa, you've already read 20th/21st Century Boys, but Monster is an amazing epic, Master Keaton's pretty good and I have high hopes for Happy. Then there's Yoshihiro Tatsumi's A Drifting Life, a history of the early alternate manga movement gekiga. Not everyone's cup of tea, but if you're interested in the history of manga, it's a must along with Tezuka Osamu Monogatari, the officially sanctioned biography of the great by one of his chief assistants. Taniguchi Jiro has a wide range of styles, but he slow, introspective works are what I'd recommend-Walking Man, A Zoo in Winter, Furari, and with more plot: Haruka na Machi e. Lone Wolf and Cub, Lady Snowblood, and Blade of the Immortal if you want samurai manga. Newer, there's Mushishi, Hotel by Boichi, Kuragehime, xxxHolic-though the anime seasons are better, Petshop of Horrors(gets better after the first volume when they get around to giving the second main character some development), and The Itou Junji Collection for horror, along with his Uzumaki. If you don't mind the truly bizarre, any non-hentai by Dowman Sayman(also spelled Douman Seiman)- The Voynich Hotel, Nickelodeon, and Melancholia, in that order, as well as his one-shots; also bizarre but good are the works of Nishioka Kyoudai; less humor and more death and depression, though. Anything based on a story by author Otsuichi is another good bet- Goth and Calling You-but he really shines as an actual short story novelist if you're willing to read books without pictures. They don't quite qualify as light novels for MAL's purposes(keep appearing and disappearing off the site at various mod's whims), but the novel versions of the earlier two, plus Zoo, and Summer, Fireworks and My Corpse are some of my favorite manga-adjacent books.
edit-Almost forgot. Barefoot Gen. The go-to about the Hiroshima bombing.