Just gonna run through some manga being mentioned here.
As great as Berserk is I feel like it's being carried by its first major saga to this day, the golden age arc. It has been a steady decline since then, not that declining from near perfection means it's not a great manga anymore (that saga is probably the best stretch of manga I've ever read), but the journey on the boat was the final straw for me to drop it, I'll come back when Berserk is concluded, never want to experience something as uneventful as that at a snail's pace.
Kingdom is definitely something that compares as far as manga concerned with large scale war, without the need to be as edgy, closer to my cup of tea.
Attack on Titan is honestly the most well planned long running series I've read, the attention to detail is insane here. It's managed to be a completely different series than what it started out as, but it didn't just come out of nowhere, it was gearing up for it the entire time, the current state of the series is its true form and all the hints were there at the beginning. It's one of those manga where you go back and are more impressed on a reread.
Jojo is great, only thing is, Araki gets away with the most bullshit writing at times simply thanks to the the pretext of being bizarre, he's set up his series the most brilliantly. He can always keep things fresh, since each part is almost completely its own thing with a different MC and a different setting. It has the potential to go on forever, and with the justification of being bizarre he avoids being critiqued like other mangaka are. He truly has it made.
I'm not sure where One Piece fits in here, it's a completely different lane like many have already stated, how well does a manga do what it intends to do? That's really the most objective way of looking at a manga imo. How am I supposed to compare One Piece to Monster? One Piece's ability to interconnect such a massive world is second to none. No series has or probably will ever come close in terms of the amount of theories generated, and it's thanks to how Oda has set his world up. I mean there are massive fanbases of characters with like 3 panels to their names, I'm not sure I've seen anything quite like it with any other manga. Oda is truly a madman dedicated to his craft.
As far as HxH goes, it has less holes than One Piece, Togashi crafts his verse in a way where he's already thought through any potential contradictions in his system, and he manages to almost never write himself into a corner despite not shying away from leaving everything bare for the reader to see (which Oda rarely does, he keeps things up in the air, leaving himself a bail out in case he ever regrets something). Compare his handling of Haki to Nen, we still don't even know what Haki really is. It's like Oda is figuring it out himself as he goes. So if you're leaning on the battle aspect of a battle manga, there are virtually none that are as sophisticated as HxH, OP doesn't even come close in that aspect.
The most complete shonen battle manga that I've read was Rurouni Kenshin, much smaller scale, not the most complex piece of writing, but it didn't aim to be. The focus never got out of hand to the point where it came at the cost of the main cast, consistent from beginning to end and wrapped everything up perfectly. Rare to see a proper conclusion for any long running series.
My personal favourite manga though atm is probably Tokyo Ghoul (though AOT might be taking over), it's probably more flawed than some of the ones mentioned above but it's the perfect blend of style and substance which is what this medium is about at its core imo. I think it's among the best when it comes to character development through trauma, many series have tried similar things but come off as fake deep instead. I have an appetite for psychology, and Tokyo Ghoul is a masterclass in psychological horror.