Is man God's greatest creation?

It's very simple. The verse comes from the Hebrew Tanakh, not English. The text says God made us a little lower than Elohim, God, Himself. We were made higher than the angels.

I'm not sure you can get a good answer to this question without opinion. I wouldn't say we are the greatest creation because everything God makes is great.

God looks to make sure that his creation came out the way he intended it. All throughout chapter 1 in Genesis you see this "Then God saw that it was good".

So really asking which creation is God's greatest leaves the matter open to a persons perception. Some of the things that could be said are the greatest are the earth, Heavens, angels, humans, emotions, music, so on.

I think from Gods standpoint, he views and loves everything equally. I can't remember any scripture that articulates any one thing as being his greatest creation or achievement.

I myself would say it is love. Because it is the platform for everything that God does.

The way I interpret this passage

Psa 8:4 What is man, that thou art mindful of him? and the son of man, that thou visitest him? Psa 8:5 For thou hast made him a little lower than the angels, and hast crowned him with glory and honour.

God is the God of all creation. Jesus IS also God who dwelt among men (John 1). He lowered his status as God himself to dwell among men and die for them. That's what it is referring to being made lower than the angels. The angels are up in heaven. Men are down here on the earth.

Phil 2:5-8 - Let this mind be in you, which was also in Christ Jesus: 6Who, being in the form of God, thought it not a thing to be grasped to be equal with God: 7But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: 8And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.

As for the specific question about is man God's greatest creation I would says yes absolutely.

God didn't send Jesus to die for Angels, or plants or animals. He didn't become a horse when he came to live among men. He became a man. We were made last on the earth for a reason. It was because in the beginning everything on the earth was made for man to enjoy. Only man is made in His image and we were the ones who were given dominion over the earth.

Gen 1:27-28 27So God created man in his own image, in the image of God created he him; male and female created he them.28And God blessed them, and God said unto them, Be fruitful, and multiply, and fill the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moves upon the earth.

I'm going to answer this question from the point of view of the Church Fathers, and in particular from the point of view of Gregory of Nazianzus, also known as Gregory the Theologian because of his enormous influence on Christian theology. Many of these themes were also picked up in the theology of Maximus the Confessor and, while they are not particularly well known in the West, they are a significant feature of Eastern Orthodox theology.

The passage in particular is Oration 38, known as On the Theophany. This is a little hard to find in decent translation on the internet, but this is one as part of a dissertation and this one is available on NewAdvent. A very good translation is that of Nonna Verna Harrison in Festal Orations. I'll quote from the first of these translations in my answer, however, because I'm currently 850 miles from my copy of Festal Orations.

In many ways, this is a commentary on the creation narrative of Genesis. Gregory sees creation as double:

• The intellectual or noetic creation: the creatures not of matter but of pure intellect. Gregory writes, "We must understand them either as intelligent spirits, some kind of immaterial and bodiless fire, or as some other nature that is nearest the beings described." (Or. 38.9) In many ways, these are the creatures we know as angels.
• The material creation: the creatures of visible, tangible matter. Gregory writes, "this is both the system and combination of heaven and earth, and the things between them, praiseworthy for the shapeliness of each, but more praiseworthy for the well-joined nature and harmony of the whole" (Or. 38.10) Whereas the creatures of the intellectual realm are like in nature to God, the creatures of the material realm are "foreign" to him.

So we have these two creations. They are good, but there is a limit:

Mind and sense perception then, having been distinguished from one another in this way, stood within their own limits, and bore in themselves the magnificence of the creator Word, being silent they praise the great work and are brilliant heralds. There was not yet was not yet a blending of the two natures, nor any mixing of opposites, a sign of a greater wisdom and generosity as regards the natures; nor was the entire richness of goodness known (Or. 38.11)

Then, God creates a new thing out of both:

Then the Creator Word, having willed to show this forth, also crafts one life from both, I mean both invisible and visible natures, the man; and taking the body from the matter already created, and from himself he added the breath of life, “which thing Scripture knows as the noetic soul and the image of God” (Or. 38.11)

This is the human being, the union of the invisible (intellectual) creation and the visible (material) creation. The purpose of man is precisely this: to unite the two creations.

He continues to describe what this man is like:

He set upon earth a sort of second cosmos, a great in a small, another angel, a composite worshiper, a beholder of the visible creation, an initiate into the intelligible, king of things on earth, subject to what is above, earthly and heavenly, transitory and immortal, visible and intelligible, a mean between greatness and lowliness, the same spirit and flesh. (Or. 38.11)

In this way, the human being is the crown of creation, uniting the whole creation in one. This anthropology has a salvific purpose as well: Oration 38 goes on to discuss Christ in his Incarnation as uniting human nature to God, and therefore as uniting all of creation to God, since all creation is united in the human being.

So in this way the human is the crowning of creation. Man is "lower than the angels", according to Gregory's account, because the angels are part of first creation, in nature closest to God.