My professor told me famous can't be used with most, because famous is already superlative, just as brilliant or excellent is . But I wonder if his speaking is true. Hope some native speaker will help me !
First of all, your professor is wrong in saying that famous is already superlative. If he thinks so then ask him, what are the Positive and Comparative forms of the adjective famous? Obviously, he won't have any answer because famous in itself is a positive adjective.
Also, if I say "He is a famous YouTuber.", you don't know how famous he is. Is he 'more famous' than PewDiePie? Is he the 'most famous' YouTuber in the locality? Is he the 'most' famous Youtuber in the world? Look! 'Famous' has so many levels. It's not like how we use Good-Better-Best. If I say "Messi is the best footballer", then he is the best footballer, and he is better than all other players in the world.
Again, if I said, "Johnny is the most famous Youtuber", then this means no Youtuber is more famous then Johnny.
Even Wiktionary says so:
famous (comparative more famous, superlative most famous)
Well known. In the public eye.
"Some people are only famous within their city."
I hope you and your professor have a good talk over this now :)