You can also say 니가 짱! ni-ga jjang or 짱이다 jjang-i-da But it's definitely informal. You can say it with family or friends.

넌(너는) 최고야 the form above is informal and for making it formal you can use this: "(person's name)+는/은 (depending on the name, ending in a vowel or consonant) 최고예요.