The highest of the three traditional angelic hierarchies, consisting of Seraphim, Cherubim, and Thrones.
The First Hierarchy contains the Seraphim, Cherubim, and Thrones. In the Dionysian and Thomistic tradition these orders are understood as most immediately oriented toward God.
Etymology
Hierarchy from Greek hierarchia, “sacred rule.”
Sculptor Notes
In sacred art the first hierarchy often lends itself to the least ordinary human imagery: concentrated wings, fiery forms, eyes, wheels, throne symbolism, and celestial ornament. The sculptor should resist making all three orders into identical human figures with different labels.
References
https://www.newadvent.org/summa/1108.htm