KCBS, The Royal is the biggest and along with The Jack arguably the most prestigious. I think that while you can certainly add Sam's Club and now King of the Smoker to the discussion they are new events and it will take some time before they reach the same level as The Royal and Jack which just celebrated their 33rd and 24th year respectively.
As far as other sanctioning bodies the Houston Livestock Rodeo and Memphis In May contests are also considered very prestigious trophies.
I'm sure I'm leaving 1 or 2 out but those are the ones that come to mind right off the bat.
"What sort of people are these charcoal masters? They behaved badly and were unconcerned with appearances. Their hair was long and unkempt and their clothes were wrinkled and old. They drank beer to and from the crab house and they made rude noises while we cooked." Tao of Charcoal