RE: The BBQ Contest
Memphis in May and The American Royal BBQ Cook-Off (which Al referred to above as the "Kansas City BBQ Society Royal Competition" both claim "World's Largest" status. In one way or another they're both right.
Memphis in May covers the most square footage and The American Royal BBQ Cook-Off has the most contestants. So depending on whether you define "World's Largest" based on physical area or the number of contestants involved, that will determine who's the biggest. Me personally, I go with the number of contestants.
None of this should take away from the legitimacy of either of these contests. Both of them, along with the Jack Daniels competition are highly regarded by the Barbecue competition community, and almost anyone you talk to in that community would consider them to be the "Big Three".
As for the declaration of a "be-all/end-all...best BBQ in the world", none of these competitions has that designation. I'm pretty sure KCBS contestants have their points totaled up at the end of the cook-off season much the same way NASCAR drivers do. No one competition will get you any grand titles. The more contests you enter and the more categories you win, the better your reputation on the circuit.
As for an actual "Best in the World" or similar title, I don't think one actually exists, even for the guy with the most points. Even winning all three of the above named competitions will not get you that title as they are all sanctioned by different governing boards.
That's all I've got in my limited information bag. Hope that helps.