To clarify I mean in popularity and legacy not standalone quality.
Throughout the mid-80's to the early-90's, Dragon Ball had taken the world by storm. Nothing before was quite like it and it was like the largest thing in Japan. It's also a major inspiration for many other titles (even YYH and HxH take some major cues from DB) with no better example than One Piece.
I had always viewed One Piece as the true spiritual successor to Dragon Ball as no series besides really gave that same feeling... that quirky, simplex and epic story but the student may have surpassed the master. The One Piece manga has outsold the Dragon Ball manga making it the current best-selling manga series, the anime gets better tv ratings than Super (which is somewhat new), and the Film Z movie grossed more than either Battle of Gods or Resurrection F worldwide in Japan alone.
So how is it... did One Piece officially take the throne or does Dragon Ball still sit on top of it?