The best thing for Mbappe is to remain at Paris Saint-Germain in my opinion. It is almost certain that he will be the only player in the history of football who will have trouble playing alongside Messi and Neymar.
In my own opinion, I do think that the best thing is for him to leave the club and go to real Madrid that is looking for his services, there he will have a better performance. just check out his performance on before when Neymar and Messi has not joined him and he was playing with only ,I guess calvani In PSG and later with Neymar, the two big shoes Messi and Neymar has overshadowed his credibility and he needs to move out where he will become his own boss in the field of play like before.