In your opinion, which primary stat is the best?

I think all skills are equally important. What's the point of having a lot of knowledge if you have almost no magic? What's the point of having a lot of magic if you have no knowledge? It all depends. Attack and defense are different. You might have great defense, but your attack could be lacking, or vice versa. So, I try to level up everything equally, focusing on what's lacking. :)