Strictly speaking, the highest nature that God has created was that of the greatest of the angels – Lucifer. The Blessed Virgin Mary, though, became the loftiest creature through her sacrifice, her works, and the grace of God (see CCC 490-493). Her exaltation was not an act of creation but of sanctification. God made Lucifer magnificent in his nature and he corrupted himself; God made Mary humble in her nature (a human being and, as such, inferior to the angels) and she sanctified herself. We can see that a sort of “inverted parallel” exists between these two figures:
–Lucifer is the most perfect creature by nature; Mary, the most perfect creature by grace.
–Lucifer corrupted himself by disobedience; Mary sanctified herself by obedience.
–Lucifer wanted to be king, refusing to serve, and in the end became nothing; Mary wanted to be nothing, desiring to serve, and in the end was crowned Queen of heaven.
There is a parallelism even in their titles – the “Angelic Star of the Morning” (Lucifer) and the “Star of the Morning of the Redemption” (Mary):
–The first star, Lucifer, fell from the angelic firmament; the second star, Mary, was elevated.
–The first star, which was spirit, fell to the earth; the second star, which was human, ascended to heaven.
–Lucifer did not want to accept the Son of God made man; the Blessed Virgin Mary welcomed Him in her womb.
–Lucifer is a spiritual being who ended up making himself worse than a beast (without ever ceasing to be spiritual); Mary is a human being that ended up becoming better than an angel (without ceasing to be human).
—from the blog post Is God’s Greatest Creation Lucifer or the Blessed Virgin Mary? by Fr. Fortea