Informal:
* Awesome!
* Rad!
* Sweet!
* Killer!
* You rock!
* You're the best!
* Nailed it!
* Boom!
* Epic!
* That's the stuff!
* Good job, champ!
More playful:
* You da real MVP! (Most Valuable Player)
* You're a legend!
* You're a superstar!
* You're on fire!
Slightly more formal:
* Well done!
* Great work!
* Fantastic!
* Impressive!
The best term to use depends on the specific situation and your relationship with the person you're speaking to.