The Comics Chronicles has just released a ranking of the top selling individual books for the decade just ended, based on sales figures/advance orders reported by Diamond Distribution.
You can see the entire list here:
Topping the ranks at #1 and with over 500,000 copies sold is AMAZING SPIDER-MAN #583 from January 2009, the Barack Obama commemorative issue. Nothing else comes close, as the #2 issue - - CIVIL WAR #2 in June 2006 - - - sold 341,856 copies.
What's significant is that CIVIL WAR dominates the upper echelon of these rankings, with all six issues of this ground-breaking mini-series crashing the top 10 positions at #2, 3, 4, 6, 7 and 9.
Also, Marvel rules as well, with 8 titles among the top 10. (The other is CAPTAIN AMERICA #25, March 2007). DC just manages to grab two slots with ALL STAR BATMAN AND ROBIN #1 (July 2005) at #8 and INFINITE CRISIS#1 (October 2005) at #10.
As you might expect Marvel and DC dominate the top rankings. You have to move down the list to position #111 to find a different publisher (Dark Horse, with BUFFY THE VAMPIRE SLAYER SEASON EIGHT #1 in March 2007 with 152,214 copies sold, a humble number compared to the others.)
Keep in mind that these are figures for direct market sales (comic book shops) only of single issues and do not include what sold through newstands and bookstores. Nor do the numbers include trade paperback collections and hardcovers. I'm sure the list would change if those statistics were available. Unfortunately, only Diamond Distribution is keeping the kind of detailed infomation needed to compile a listing like this. I would love to see some details from Barnes & Noble, Borders, etc.
As I think back on the last ten years - - if I had to narrow everything down to one single mini-series or title that defined the decade - - I would probably choose CIVIL WAR. It's certainly worthy and had a major impact on what followed in super-hero books, both at Marvel and DC.