Theists: What do you mean when you say that God is 'perfect'?

[Neo-Scholastic]

I don’t know if it makes any sense to consider God a species within some other genus. My place-holder thought is that God serves as maximally great all-encompassing category like Plotinus’s mystical notion of “The All” or from the Book of Revelation, “the All in all.”

I hope you're not defaulting to the supposition that God is "all things to all men" -- that God embodies all perfections that could possibly be.

That is two contradictory concepts in one sentence. The notion that God is “all things to all men” is the exact opposite of the notion that God embodies all perfections.
The first allows someone to posit things like ‘perfectly evil’ or ‘completely ugly’. Personally, I have no idea what kind of theory of value would allow someone to contrast perfect good with perfect evil without contradiction. Perhaps you have something in mind.

In the Scholastic tradition, evil is considered a deficiency, a “lack of the good that ought to be there,” to paraphrase Augustine. A basketball is definitely more of a perfect sphere than say a loosely packed snowball. The pieces of a tangram puzzle are more perfect triangles than either a yield sign or a spanakopita.

Similarly, human morality consists of more perfectly manifesting the virtues of humanity. An insane person lacks the rationality that is natural to being human. A sociopath lacks the empathy that is natural to a human. A thief lacks conscience, etc. The difficulty of flushing our virtue ethics is determining what exactly the natural and normative virtues of humanity are. That doesn’t bother me too much. The debate about what those virtues are only makes sense after mutual agreement that there actually are such virtues.

As noted previously, theists assert that our values cannot and do not come from the universe itself.

If the existence and nature of the physical universe is contingent then any values or principles derived from observation of the physical universe ultimately trace back to some non-contingent. This belief is not mere assertion; but rather, a logical consequence of an Unchanged Changer, First Cause, and Necessary Being.

If a theist is to justify why God is all loving, and that itself is supposed to be a desirable thing to be, the only place we can turn for that bias is to God himself. But then God's perfections become simply those things which God himself considers to be desirable to be perfect in. We end up with a curious construction in which God's supposed greatness rests upon what he himself considers great.

That’s like someone saying that the standard for a perfect sphere is whatever he considers it to be like. If he thinks popcorn balls are the standard by which all spheres should be judged, that leads to the bizarre conclusion that popcorn balls make better spheres than a glass marbles. God is not a post-modernist.

It isn't so much that you decry the lack of a foundation in alternate moral theories. They have their own suppositions about the ultimate good -- nature, our species, society, well-being, the planet -- you simply disagree on what the highest good actually is, and whether there is any foundation for considering the specific highest good to be rationally justified. If I'm at all familiar with your arguments, it is that you believe these 'highest goods' are not ultimately justified, whereas God as the highest good supposedly is justified. For the reasons stated above and others, I think that your highest good is no better justified than any of the alternatives. You simply believe it is. And when I asked for your reasons in the Euthyphro response, I believe you said, "[It] just is." How you think that any less arbitrary than any of these other highest goods is a mystery to me. It only becomes worse when I observe that the existence of your highest good is based essentially upon bare assertions made by mortal men scattered throughout the dark recesses of history.

As I feared, we appear to be talking past one another yet again. Values based only on contingent things do not satisfy the criteria for a highest good, i.e. that of resting on absolutes, and as such any source of value other than one tracing back to an absolute is arbitrary. This has been the reasoned opinion of thoughtful mortal men since the dark recesses of history and on through modern thinkers like Nietzsche and Sartre. It is a contemporary conceit that people who lived in by-gone eras are any less wise or rational than today.