@choperman:
i just posted this let's see how the naruto and bleach fans reply to logic
http://answers.yahoo.com/question/index?qid=20100221150744AAagTTx
One Piece has outsold everything.
So. Thats the "justified" answer you're probably looking for. Total sold, first run print numbers, ammount sold annually, quantity sold per time its been out, per how many volumes it actually has right now, etc. You can point to solid numbers and arrive at OP as the answer pretty handily. (It surpassed Dragonball last year, and Dragonball had nearly 20 years head start on it.) And the numbers in Japan are the only ones that really count where manga sales are concerned, since they go into the millions, whereas stateside even high printruns generally only go into the ten thousands.
But then, Avatar is the highest grossing movie of all time, and that doesn't mean shit. Gone with the Wind is the highest selling ticket number movie of all time, and that doesn't mean much either. Its all subjective. You like what you like and if you need numbers to prove why something is good, thats a completely halfhearted reasoning. All that does is support the creators and ensure you get more, which are good things, but it has no bearing on actual quality or "bragging rights."
Twilight set box office records last year. Dwell on that for a while.