You'd be hard pressed to find Manga that are better and the list is definitely very short. I feel that anyone who has read Berserk will say Berserk is better - objectively that is a masterpiece.
I would also personally rate Vagabond, Monster, Oyasumi Punpun above One Piece too. I also find Onepunch Man more enjoyable overall although the quality of the story is self-evidently nowhere near what One Piece offers it's readers. I've read over 100 Manga, i've actually lost count, but One Piece is definitely near the top. IMO:
1. Berserk
2. Vagabond
3. Monster
4. Oyasumi Punpun
5. One Piece
6. Great Teacher Onizuka
7. Kingdom
8. Death Note
9. Shingeki no Kyojin
Torn for 10th but either Hunter x Hunter, Jojo's (esp. Part 7 + 8), Gantz, Inuyashiki or Uzumaki
Except this is all subjective - One Piece is definitely up there but honestly it just is not on the same level as Berserk, Vagabond, Monster or Punpun.
Berserk, Vagabond and Monster are good calls, but like I outlined in my other thread:
What does OP offer that any other "generic" shonen (how OP fans call them) like MHA, Naruto etc. doesn't offer?
Only real answer is always "well, its longetivity and Oda's work ethic".
That's about it.
Yes, it has 1053+ chapters, but in terms of actual storywriting it's as generic and basic as it gets.
Each arc has the exact same blueprint and you know exactly how it will end in the grand scheme (I also outlined that blueprint in the other thread). It's just really repetitive and not creative at all in terms of general storywriting.
That's also why I personally would never rank it as high as Death Note, HxH or Jojo.
OP for me falls right into the Naruto, MHA, DBZ bracket in terms of writing.
OP has other strengths, but in terms of storywriting it isn't close to Death Note etc. (and yeah, I won't even start with Berserk etc., that would be ridiculous).