Formal:
* Jesteś najlepszy/najlepsza. (You are the best) - This is the most direct translation and is suitable for formal situations.
Informal:
* Jesteś the best! - This is a more casual way to say it, using the English word "best".
* Jesteś super! (You are super!)
* Jesteś świetny/świetna! (You are great!)
* Jesteś zajebisty/zajebista! (You are awesome!) - This is very informal and slangy.
The best choice will depend on your relationship with the person you are speaking to.