Play With the Numbers
Yes, I'll find any excuse to post pictures of Aoi Miyazaki.
On the back cover of Nana Vol. 3, Viz proudly trumpets the series as "The all-time best-selling shôjo title in Japan!"
... Except, it isn't.
Comipress has a mostly definitive all-time shôjo sales list, kindly compiled and translated from the research of various Japanese fans. Going by total overall sales, Hana Yori Dango (Boys Over Flowers) is the WINNAR! with 54 million copies sold.
Nana is close, but no cigar, in 4th place with 34.5 million copies sold.
If you go by average sales per volume, then Nana DOES win with 2.3 million per volume. But that's a rather shifty way of defining the "best-selling of all time" ...
Obviously, with a list like this, it's weighted towards series that are very long-running, complete, and old enough to have picked up cultural relevance. Something like Patalliro, a popular re-telling of The Journey to the West, gets in by virtue of being, oh, 76 volumes long.
Other interesting factoids from this list...
Fantasy epics Red River, Please Save My Earth and Basara all clock in very close to each other, coming in 12th, 13th and 16th. In between is the all-time classic The Rose of Versailles at No. 14.
In the Magical Girl wars, Cardcaptor Sakura narrowly beats Sailor Moon, both selling about 12 million total but with Sakura at No. 21 and Moon at No. 23.
North American favorite Fruits Basket is just a bit below, at No. 25.
Then you get down to the stuff that's more recent, and therefore has sold less because it's not finished yet, or ended up not having that many volumes. Two of Tokyopop's dropped licenses, Kodocha and Marmalade Boy, picking up No. 30 and 31. Yazawa's next most well-known series, the 5-volume Paradise Kiss, at No. 40. The currently very popular Honey and Clover is at No. 42. Other contemporary hits like Nodame Cantabile and Ouran High School Host Club aren't far behind, at Nos. 46 and 47 respectively. And by the time you've dug down far enough and don't care anymore ... there's Socrates in Love, the highest ranking single-volume work at No. 76.
And yeah, I've dug down far enough that I don't care anymore. :P
But Nana. Intensely popular? Yes. Best-selling of all-time in its field? Aaaaaalmost.