Formal:
* C'est formidable! (This is very common and translates literally to "It is formidable!")
* C'est extraordinaire! (This means "It is extraordinary!")
Informal:
* C'est génial! (This is a very common and casual way to say "It's awesome!")
* C'est super! (Similar to "It's super!")
* C'est top! (This means "It's great!" or "It's top!")
* C'est cool! (This is a very casual way to say "It's cool!")
Slang:
* C'est le pied! (This literally means "It's the foot!" but is a slang expression meaning "It's awesome!")
* C'est la classe! (This literally means "It's the class!" but is a slang expression meaning "It's awesome!")
Other options:
* C'est incroyable! (This means "It's unbelievable!")
* C'est fantastique! (This means "It's fantastic!")
The best way to choose the right expression depends on the situation and your relationship with the person you're talking to.