When somebody asks you "How are you?", there are many different ways to say that we feel well.

I'm fine thank you.
I feel great/marvelous/fine.
Couldn't be better.
Fit as a fiddle.
Very well, thanks.
Okay.
Alright.
Not bad.
Much better.
All the better for seeing you.

Explanations

• Many students only learn phrase 1. Try to learn some more expressions.
• Phrase 2 is very positive. Marvelous is probably the strongest.
• Phrase 3 is saying that it is impossible to be more healthy/well.
• Phrase 4 is an idiom that says you feel very fit and healthy.
• Phrases 6, 7 and 8 are positive but not strong. People sometimes say these when they don't feel very well but they want to be polite.
• We say phrase 9 when we were sick before and now feel (more) well.
• Phrase 10 is the friendliest phrase. We are saying that seeing the other person has made us feel healthier and happier. Don't use this with strangers.