Couldn't be better.

Very good.
Pretty good.
Good.
All right.
Very nice.
Nice.
Very well.
Well.
Very fine.
Fine.
Not bad.
Not to bad.
So-so.
Not fine.
Not well.
Not good.
Bad.
Pretty bad
Very bad
Terrible.
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Are all expressions can be used as the responses to the question, "How are you?"
Are there any expressions which can not be uses as appropriate responses?

3 answers

Yes. Do you need examples?
Thank you. Where are you from? Are you a moderator on this website? Where can I find sych examples on the Internet?
Most people stick to the one-word responses!

Good.
Fine.
OK.
Awful.
Terrible.
etc.

Sometimes a two-word response will slip in!
Not good.
Pretty bad.
etc.