From what I've heard, Lawrence Kasdan, Tom Stoppard, Carrie Fisher, and quite possibly Quentin Tarantino, among others, contributed significantly to the prequel scripts. Likewise, Steven Spielberg, Quentin Tarantino, Robert Rodriguez, and others, supposedly directed key sequences of the prequels.
Lucas is more than willing to let others play in his sandbox, and almost always gives credit where credit is due. And when he doesn't, there's usually a good reason. (Union/Guild conflicts). He usually shuns the limelight, but will cheerfully step into the public arena to let others poke fun at him, or to sing the praises of another.
From where I'm sitting, the egomania he's often accused of having is a myth with little basis in reality.