I saw a preview for the new direct to DVD Superman cartoon, The Death of Superman. In the promo, it says it is based on the “best selling graphic novel of all-time.”
That surprises me. Excusing the distinction between “graphic novel” and “trade paperback,” I would never have guessed The Death of Superman would have held that honor.
If you’d asked me that question, I would have answered Watchmen with some confidence. If not Watchmen, something like DKR or Kingdom Come.
While I wouldn’t be surprised to find that it was the top seller–only including DC comics as of 1999–I doubt that it’s the true number one. Manga have been topping the list for several years according to a quick google trying to find such a list–so probably one of them is it.