Which one is correct? Leaning towards first one but I am unsure because a more basic version would be "How good is it?"
Think of it like this. Work backward from the answer:
You think it is X good. ... replace X with interrogative 'how' You think it is how good. ... invert the subject with auxiliary 'do' Do you think it is how good? ... move the interrogative phrase to the front How good do you think it is?
"How good is it?" works backward from a different answer:
It is X good. ... replace X with 'how'. It is how good. ... invert with auxiliary. Is it how good? ... move the interrogative to the front. How good is it?
But your question isn't about how good it is, it's about what the other person thinks.