Hachim Mastour should be the best,he's amazing with tha ball at his feet,he's highly skilled and ambitious.He should be in top3.
Tielemans is the best U19 player in the world,key-player for Anderlecht and he scored some fantastic goals.
Also,Gabriel Barbosa is already a star in Brasil,he's tuted as next Neymar and he's scoring pretty goals.
Adama Traore was very underpowerred in this year FM,although he left Barca,his talent is immense,best young player from La Masia.
Sergi Samper,Sandro,Odegaard,Oliver Torres,Angel Correa,Lee Seung-Woo,Ianis Hagi,Cristian Manea are some players already counted as wonderkids in FM15 ,but future wonderkids for FM16.