All three sets can go enhanced, but the "Classic AI" and "Expanding" will not try to use all the units. You have to really define "best"... as the interesting phenomenon is they are "different", which is really cool in that the same approach does not work as well with a different AI, and you could find you are actually in trouble in some games. Don't just think one AI fits one set.
The AI's are limited, but there is definitely room for improvement. But the community is really going to have to encourage AI development to make it happen.
My hope is we will get further AI's with different personalities...maybe even one that has distinct play styles inside it.