Informal:
* Super! (This is a very general exclamation of approval)
* Gut gemacht! (Literally "Well done!")
* Prima! (This means "excellent" or "first-rate")
* Toll! (This means "great" or "wonderful")
* Geil! (This is very informal and slang, meaning "awesome" or "cool")
Formal:
* Sehr gut! (This means "very good" and is more formal)
* Ausgezeichnet! (This means "excellent" and is the most formal)
* Hervorragend! (This also means "excellent" and is very formal)
Other options:
* Das hast du super gemacht! (You did that super well!)
* Tolles Ergebnis! (Great result!)
* Klasse! (Classy! or Excellent!)
The best choice will depend on your relationship with the person you are speaking to.