As to the "most popular" dog breed, it would be awfully difficult for anyone to say with certainty if "pit-bull mix" dogs are or are not the most numerous, 'cause frankly, the majority of "mutt" owners in general don't have any means of being tracked.

From looking at the AKC registrations, you can see that the following dogs that are commonly called, "Pit Bulls," by the previously defined stupid Americans are:

Boxer - #7
Bull Terrier - #61
American Staffordshire Terrier - #66
Staffordshire Bull Terrier - #80

AKC 2006 registration rankings

There's no ranking for "American Pit Bull," because that's not a breed the AKC recognizes, so all of the rankings that are out there, and based on the AKC rankings are useless. An Amstaff is different than a APBT, even though some dogs can be registered as an Amstaff to the AKC, and a APBT to other registration organizations.

I would also go with the assumption that the majority of "pit bull" owners are not registered with the AKC, or any association for that matter... It is not usually a purebreed & registered APBT or Amstaff that is involved in attack incidence.