Formal:
* Bardzo mi się to podoba. (This translates directly to "I like this very much.")
* Jestem bardzo zadowolony/zadowolona z tego. (This means "I am very satisfied with this.")
Informal:
* Bardzo mi się to podoba! (More emphatic and informal version of the first formal option)
* Świetnie! (Means "Great!" or "Awesome!")
* Uwielbiam to! (Means "I love it!")
Other options:
* Lubię to bardzo. (A more literal translation of "I like it very much.")
* To jest świetne! (Means "This is great!")
The best choice will depend on the specific situation and the relationship between the speaker and the listener.