For me I would say that as far as Shounen goes, nothing beats One Piece for me. One Piece's world building is top tier, and I can't think of any other manga off the top of my head that does it as well as One Piece. There are a lot of manga out there that are good at their specific genre or themes like: horror, tragedy, slice of life or mystery etc. But for me the world of One Piece, the four Blues, the Grand Line, the New World, everything is detailed and lived in, and reading it you get immersive and sucked in like no other comic or manga.
However that doesn't mean that I think that there aren't other good manga out there.
Berserk has a incredible dark art style, and a beautiful horrifying world
I also really like the world building in Magi: The Kingdom of Magic. It has pretty good world building, not on the level of One Piece of course. The stories are beautifully written too.
Attack on Titan was hot for a time when it first came out, I first got to know it through the anime, the manga sadly isn't top tier when it comes to art, but still good. Attack on titan started off really cool and intense, then towards the ending I started disliking Eren and some of the plot points with the whole 'chosen one or destined' thing kinda took me off. They also killed off so many fan favorite characters.
A lot of people on here like Kingdom a lot, and I agree its amazing, but weirdly for me as a history nerd the Warring States stories aren't new to me, and I've already read something similar in Chinese manhua comics in the past that I randomly found on the internet one time lol.
I hear Hunter x Hunter is also good, but its also known as Hiatus x Hiatus, so maybe i'll read it once its all finished.