How to find the best constellation of 16QAM?

First, the optimal 16QAM. According to my understanding, the best QAM must be Gray code. I forgot exactly where I saw the paper. 8QAM has no gray color and is easy to verify. Square is definitely better than ring, which can be deduced from Euclidean distance between symbols. You can try. It's not difficult. Let's deduce the decimal system from Gray.