How should I say "I feel very good/I'm very fine" in response to 你好吗?
There is "我非常好" I'm extremely well, or I'm extremely good.

There is also "我非常开心" I'm extremely happy.

Maybe if you want to aim for something a little more subtle you could try "我很开心" I'm very happy.

If you are mentioning that you are this "good" you will likely be asked for an explanation!