One Piece sold very well, so it must be good. Going by that logic, we can also say Justin Bieber's albums sold very well, so they must be good. Is what most people would use as the counter argument. But, there is one big difference between them, that is longevity.
One Piece has existed for 17 years and always consistently going strong throughout the run, unlike Justin Bieber's songs. Many of the records broken by Justin Bieber are all thanks to the fanaticism of the fanboys/girls and are irrelevant to what makes a music good (such as most viewed video on Youtube, or most played in radio in the first week). You might argue that music gets boring after hearing for too many times. Yes, it's true, but good music, while get listened less often than before, will never totally disappear, just like that Titanic's song, Michael Jackson's, and so on, while Justin Bieber's song vanished in few months.
Back to One Piece, while One Piece started in 1997, there was still Rurouni Kenshin going on in the magazine. Judging by the time it ended, it should be at the Remembrance arc aka the best arc in the whole series at that time. Despite that, One Piece could steal the first rank spot in the magazine from it for more time than it is stolen. What's more impressive is, there is no way the domination is caused by the fanaticism of the fanboys as Rurouni Kenshin definitely had more fans at the time. Well, it could be because Jump purposely promote One Piece to gain new readers since it's a new series, but while One Piece was going, came Hikaru no Go, Hunter x Hunter, Shaman King, Naruto, Prince of Tennis, Yugioh, Bobobo that budged One Piece's throne for a while, but none really beat it overall.
My point is, sure, One Piece's massive sales right now is because of the amount of fans it has right now, but the fact that it could beat the already popular series (Rurouni Kenshin), and made sure that no slightly newer series (Hikaru no Go, HxH) could take its top spot in the Jump ranking surely means something else.
There are series hugely promoted by Jump but fail, like Toriko.
There are series going strong at the beginning but slowly degrading, like Bleach.
There are series going long but never really boom, like Ippo.
But One Piece, hugely promoted, going strong and never degraded, and booming worldwide.
Well, the greatest? I don't know. A bad manga? Absolutely not.