It doesn't need to have a great ending to be the best shounen. In the post time skip and with Wano the quality is not the same as the pre time skip but the pre time skip quality is already top notch shounen wise, only HxH topples it, Naruto or Bleach are always below One piece and about modern contenders honestly I don't see nor hear any valid one. Maybe MHA or devil slayer beat it sale wise but can honestly compare to OP as a whole? I don't think so.
Aside from Bleach and Naruto the contenders should be HxH and Dragonball. But HxH, despite being great and keeping his quality, is way shorter and Togashi, sadly, could not work on it how much he wanted to and most likely HxH will never be finished in a satisfying way (maybe rushed like the other great work from Togashi: yu yu hakusho). Dragonball is probably even more widely known than One piece but it peaks with the sayajin and Namek saga, after that it is a continuous fall and everything be it GT, Super or whatever is a black hole quality wise (GT is even way better than Super and that tells a lot on how bad Super is). One piece on the other hand is very long, the quality is great for a good portion of the manga and nowadays still manages to stay good (the last chapters are entertaining) so, imi, it just needs a satisfying end to keep the pole. But if it is some shitty ending like Naruto then it could even fall. And of course no ither arcs like Wano or WCI to a degree and less Luffy Piece, please.