I'm looking for manga series that are cult classics. Rather than explain what i mean i thought it'd be best to give few examples. It can any genre but it must have some emotional or narrative depth, it shouldn't be shallow.

Examples: G senjou Heavens door, Kyou Kara Ore Wa, Ai Ren, Alive: The Final Evolution.

G senjou is one of the most emotionally resonant manga i have ever read.
Kyou kara ore wa is by far the funniest
Ai ren is one the most heart wrenching
Alive...one of the most underrated shonen manga.
hope it isn't too diffucult to reccomend.

I don't know if they're considered a cult classic but here are some that comes to mind:

If you don't mind the art, try Saishuu Heiki Kanojo
20th Century Boys
Monster
Kiseijuu
Please Save My Earth
Gokusen

This manga isn't finished yet but I totally recommend this: Saint Onii-san

your recommendations were on point 😀 but unfortunately other than 20th century boys I've read all of them. 😢

not sure how cultish any of my recs is, but since Adorbs mentioned Parasite, perhaps Oyasumi Punpun (or anything by Inio Asano) would be to your taste
I haven't read it yet, but I Am a Hero seems to have the same tone as the ones previously mentioned
Holyland
Kokou no Hito
Akira

here are also some of my personal recs. Most of these are too recent to be considered classics, and they're pretty different from each other in theme and tone, but I think they are all really good quality
Annarasumanara - really good coming of age story
Ran to Haiiro no Sekai - really good coming of age story
Fushigi na Shounen - this one needs some love
Historie - same author as kiseijuu, but it's historical fiction (and better, imo)
IS: Otoko demo Onna demo nai Sei - dunno if you like stuff like horou musuko
nobunaga no chef - historical fiction
peak - like kokou no hito, but I like this one better
Shimanami Tasogare - deals with gender issues
Song of the Long March - has been getting more (deserved) love lately, but I still want to spread some luurve
Shoukoku no Altair - similar to Song of the Long March, really interesting strategy
Takemitsu Zamurai - dunno if you read and liked mushishi, but here
Vinland Saga - vikings. probably not cultish, sorry

Not sure if I'm in tune enough with the fan community to identify cult classics, but here are some guesses:

Ashita no Joe
Homunculus
Mars
Slam Dunk
Rurouni Kenshin
D.Gray Man
Hana Yori Dango
Fukuyadou Honpo

Definitely Akira as suggested by the previous poster... Mother Sarah by the same mangaka is a good one too but I don't think it's well known.

Planetes is reminiscent of Cowboy Bebop, cult classic anime 🙂

I think Golden Kamuy and Dungeon Meshi are rapidly becoming cult classics. (They're really good too!) Though I'm not sure if they necessarily have the serious thematic/emotional theft you're looking for, since they're, well, comedies first and foremost.

Also...
Blade of the Immortal
Bokurano (and everything else Kitoh Mohiro has written)
Claymore
Clover (Clamp)
Eden - It's an Endless World!
Eternal Sabbath
Franken Fran
Gunslinger Girl
Innocent (Sakamoto Shinichi) and Innocent Rouge
Nickelodeon
Otoyomegatari
Shin Angyo Onshi
Yu Yu Hakusho (and of course Togashi Yoshihiro's superior follow-up Hunter x Hunter, but I'm not rec'ing that one until it's finished)

Seconding the recommendation for Asano Inio stuff. If you just want to try out his style, I recommend What a Wonderful World! Otherwise, his current Dead Dead Demon's Dededededestruction goes some really interesting places with the whole "alien invasion" concept.

Also seconding Historie, Shoukoku no Altair, and Vinland Saga, and if you're interested in more historical manga, try Cesare. It has one of the best handling of politics I've ever read.

If you're interested in mystery with an emotional heft, I would recommend Shirodaira Kyou's work: Spiral, Vampire Juujikai, Zetsuen no Tempest, Invented Inference, etc.