Formal:
* C'est magnifique! (This is magnificent!)
* C'est très beau! (This is very beautiful!)
* C'est très bien! (This is very good!)
Informal:
* C'est super! (This is great!)
* C'est génial! (This is awesome!)
* C'est top! (This is top notch!)
* C'est cool! (This is cool!)
* Ça a l'air super! (This looks great!)
More specific:
* C'est bien fait! (This is well done!)
* C'est bien pensé! (This is well thought out!)
* C'est vraiment original! (This is really original!)
You can also use a phrase like "J'aime beaucoup" (I like it a lot) to express your appreciation.
The best way to choose the right phrase is to consider the situation and your relationship with the person you're talking to.