Depends on what you prioritise when it comes to manga. If you want an interesting narrative then OP will likely be considered one of the best.
But if you're focused on:
Great characters
Meaningful and continuous character progression,
Complex fights,
Hilarious comedy
Dynamic panelling
Strong execution of themes,
Heart moving emotion,
Communicative character designs
Then OP definetely isn't the best. And for some of these it's not even close. Many of these areas having greatly declined as the series went on despite originally being some of the strongest points about the manga.
There are a lot of manga you can point to that excels at each of these, but as an overall package, series that do it the best, and downright perfect in some areas would be: Kingdom, Haikyuu, Berserk, Attack on Titan. All of these are leaps and bounds above OP in quality which has been falling towards Naruto tier for a while now and can easily end up being worse considering Wano and how well Oda sticks the landing of the manga.
If looking at Jump's current line up, then OP isn't even in the top 5 most engaging series personally. But ultimately what you're looking for depends on personal preference, so there is no definitive "best manga", but some are certainly better than others in many areas.
