Like others have said already, Mufasa's Ghost is my choice, too. I see it as the turning point of the film - it is the part where Simba finally decides to take his destiny seriously and stop running from his past. In a way it is the completion of Simba's character arc, since as we all know he goes back to Pride Rock after that, and defeats Scar with a surge of determination given to him by this encounter (and the fact that he now knows the thing he has spent his whole life feeling guilty about wasn't even his fault, after all). Though, really, if one of my relatives appeared to me in a giant-ass storm cloud I'd probably do whatever they told me to do, too
