what are the manga series that are legendary? series like vagabond, one piece, naoki urasawa's monster... you get what i mean?
i hope that makes sense, all answers are appreciated :)
It would depend on the person you're asking. For instance, I don't believe there are too many manga which are legendary, except maybe the long-runners like Detective Conan and One Piece, and the ones I personally cherish.
Common opinions around AniList and the anime community at large, I would think (based on the threads I've seen and the people I've interacted with) include Berserk, One Piece (again), Oyasumi Punpun and Monster. If you want to go slightly newer than that, you'd look at things like Kimetsu no Yaiba, Chainsaw Man, Jujutsu Kaisen, BnHA and Attack on Titan.
^Note there are basically no shoujo in what I've listed, because the only shoujo manga which are generally seen as "legendary" are genre-definers like Sailor Moon or Cardcaptor Sakura, and a lot of it comes out of the pages of Weekly Shonen Jump and its ilk because that pulls the biggest numbers.