Depends on the kind of BBQ you are cooking...
For Pork the most prestigous is Memphis in May as that is what MIM focuses on. Ribs, Shoulder, and Whole Hog. It is certified as the largest Pork BBQ Contest in the World. This make the winner of Memphis in May the World PORK BBQ Champion
For Brisket it is the Houston Rodeo. Nobody does Brisket like Texas and this is the biggest, baddest Comp in the Lonestar State. You win this and you are the KING of Beef BBQ.
For KCBS it is the Royal. As KCBS is a style adapted all over the USA due to it being the premeire and most used sanctioning organization, this means you are the top amongst 5500+ BBQ Teams in the organization. You are the KCBS World Champion
Now that brings me to the Jack. The winner of MIM is invited to the Jack, the winner of the Royal is invited to the Jack, pretty much gotta be a Grand Champion at one (more times than most many) regional contests. Since the Jack is, unlike the Royal, composed of winning teams from different sanctioning organizations besides KCBS, I would argue that this makes the Jack Grand Champion the Overall BBQ World Champion as you have to be able to cook many styles of BBQ well in order to win.
Like I said, Just my view
"We're a drinking team...with a BBQ problem" http://www.toosaucedtopork.com