I absolutely disagree that you can say one series is objectively better than another. Even if you can, maybe (and I also find this possibility at least questionable) point to certain elements of a work and say they are, beyond a shadow of a doubt, better than others, they are still just that - an element, a part of the entire work. You still have to judge, subjectively, how much each aspect matter for each series and how they connect to the whole - it depends on what you brings with you, which critical lens you are applying to the piece. There's no definitive rules for art criticism and a lot of ways you can judge a work. Art is so multifaceted that I don't believe there's anyway you could come up with a definitive way to judge the quality of a work, let alone compare one to another.
addendum: That goes both to who's saying One Piece is the greatest series ever written and who's saying is categorically not. I'm not just fanboying here this time, heh.
addendum two: What I've said doesn't mean that every argument is equally valid! If you have a more robust justification for why you think a series is better (Exaggerating: if someone makes a thematic analysis of character arcs and the other person says "but that punch was really cool, therefore this series is better!", those aren't equally valid points! But a better-argued point is still subjective.)