Formal:
* "I'm fine, thank you. And you?"
* "I'm doing well, thank you. How are you?"
* "Very well, thank you. How are you doing?"
Informal:
* "I'm good, thanks! How are you?"
* "Pretty good, thanks. You?"
* "Not bad, how about you?"
More specific:
* "I'm having a great day!"
* "I'm a little tired, but otherwise okay."
* "I'm really busy at the moment, but I'm managing."
Humorous:
* "Still alive and kicking!"
* "Just trying to survive another day."
The best response will depend on the context and your relationship with the person asking.