Is "le plus sympa garcon" correct?

Question

"Je suis le plus sympa garçon de l'école."
This is another example (like the earlier "C'est la plus rapide voiture du monde") that kwizik says is correct, but both my french speaking friend and Gemini says is incorrect . Could someone please check and confirm?
There's another one:
"C'est la plus grosse étoile de la galaxie."
Thanks

Céline Pickard · Accepted Answer

Bonjour Patrick,
Thank you for your query! You are right that in the instances of "sympa" and "rapide", the order is not the best. However, you might hear people say it that way, although grammatically, it is not the best option. In fact, here, what takes precedence is the position the adjectives usually take next to the word they refer to (i.e. before or after an word):
 1. Adjectives usually placed after a word:
un garçon sympa -> Je suis le garçon le plus sympa de l'école
une voiture rapide -> C'est la voiture la plus rapide du monde
2. Adjectives usually placed before a word:
une grande ville -> c'est la plus grande ville de France
un petit château -> c'est le plus petit château de France
We agree with your comment and this lesson definitely needs to be reviewed. It is on our to-do list. Please bear with us until the changes have been made.
Merci de votre excellent commentaire !
Bonne journée !

Patrick Kirk · Answer

Hi Celine, thanks for the response. Usually Kwiziq tends to be strict with grammar, so I wasn't sure if this was a colloquial or grammatically correct construction. This clears it up.