I cant answer that and neither can anyone else, truthfully. This is because to answer it fairly all the pretty girls worldwide would need to line up. Many very pretty girls will never have sought fame so will not have been seen.
Even if they could be added to the line and be judged by a panel. then that panel would disagree about their favourites, some preferring redheads, some brunettes. It's wisely said that Beauty is in the eye of the Beholder.
If Selena Gomez's lovely looks with her symmetrical features and beautiful dark eyes IS the prettiest in the world, does that bring you any nearer her if you're a bloke, or make you look any nearer like her if you're female? Find the beauty in the people important to you, for that's what makes the world turn round.
Sadly, as Selena Gomez has proved, being a beauty does not bring happiness or personal security or good health.