Formal:
* Napakaganda (literally means "very beautiful")
* Napakasaya (literally means "very happy")
* Napakahusay (literally means "very good")
* Kamangha-mangha (literally means "amazing")
Informal:
* Ang galing! (literally means "how good!")
* Astig! (this is a slang term for "cool")
* Ang ganda! (literally means "how beautiful!")
* Ang saya! (literally means "how happy!")
* Ang husay! (literally means "how good!")
The best translation depends on the specific situation. If you're describing something that is impressive, "kamangha-mangha" or "napakahusay" would be good choices. If you're describing something that is fun or exciting, "ang saya!" or "astig!" would be more appropriate.