Formal:
* Vous êtes le/la meilleur(e). (You are the best.) - This is the most formal way to say it, and is used when addressing someone you don't know well or someone older than you.
* Vous êtes vraiment excellent(e). (You are truly excellent.) - This is a more formal way to express appreciation for someone's skills or qualities.
Informal:
* Tu es le/la meilleur(e). (You are the best.) - This is the informal way to say it, and is used when addressing someone you know well, like a friend or family member.
* T'es vraiment top ! (You are truly top!) - This is a more casual and informal way to express admiration.
* C'est toi le/la meilleur(e) ! (You are the best!) - This is a very familiar way to say it, and is often used in a playful or affectionate way.
Other ways to express the same sentiment:
* Tu es formidable ! (You are formidable!)
* Tu es génial(e) ! (You are awesome!)
* Tu es incroyable ! (You are incredible!)
* Tu es un(e) champion(ne) ! (You are a champion!)
The best way to choose which phrase to use depends on the specific situation and your relationship with the person you're speaking to.