Formal:
* Вы лучший. (Vy luchshiy.) - This is the most formal way to say it, suitable for someone you don't know well or in a professional setting.
* Вы самый лучший. (Vy samiy luchshiy.) - This is another formal option, emphasizing the uniqueness of their being the best.
Informal:
* Ты лучший. (Ty luchshiy.) - This is the informal version, used with friends or family.
* Ты самый лучший. (Ty samiy luchshiy.) - This is the informal equivalent of the formal version, emphasizing the uniqueness of their being the best.
More affectionate:
* Ты у меня самый лучший. (Ty u menya samiy luchshiy.) - This is a more affectionate way to say it, meaning "You are the best for me."
* Ты просто супер! (Ty prosto super!) - This translates to "You are just super!" and is a very informal and enthusiastic way to say "You are the best."
The best way to choose the right phrase depends on the context and your relationship with the person you are talking to.