Formal:
* Muito bem! (This is the most common and versatile way to say "well done" in a formal setting)
* Parabéns! (This is used to congratulate someone, meaning "Congratulations!")
* Excelente! (Means "Excellent!")
* Ótimo! (Means "Great!")
Informal:
* Muito bom! (Means "Very good!")
* Show! (This is a slang term meaning "Amazing!" or "Great!")
* Demais! (This is also slang meaning "Awesome!" or "Too much!")
* Legal! (Means "Cool!")
The best way to say "well done" depends on your relationship with the person you are speaking to and the situation.