You wanted LeBron James in a dunk contest? You got it!
Unfortunately, it's not a real dunk contest. It's part of a cool Nike campaign about challenging yourself and he doesn't come on until about 1:10 into the video.
You only get to see one dunk from him against a random person as the voiceover says, "Hold on; we've been waiting for this." LeBron goes through his own legs, tosses it off the backboard for a two-handed dunk. It would probably register 8s or 9s from the judges at an All-Star weekend.
Maybe we haven't been missing much after all?