Mbappe will be best in the world in 2-3 years (or 2nd best to Messi) if he avoids injury and keeps his head screwed on.
I don't think Neymar will ever be the best player in the world, maybe for a very short window. He'll never reach Brazilian Ronaldo or Ronaldinho level let alone be in the same league as Messi.
Neymar has set his career back going to France in his peak years (26 now and not a player who is likely to be great into his 30s given his lifestyle).
The only thing that could hold Mbappe back is if he's stuck in France for ages. Verratti should be best in the world in his position but needs to go to Barca or Real to be tested properly.
Barca should go all out for Mbappe over the next couple of years. Messi could pass the baton on to him like Ronaldinho did for him.