Seraphim are traditionally regarded as the highest order of angels in Christian angelology, and Isaiah describes them with six wings.
The source is Isaiah’s vision of God’s throne. Each seraph has six wings: two cover the face, two cover the feet, and two enable flight. The beings call to one another with the thrice-repeated “Holy,” while one seraph purifies Isaiah’s lips with a live coal from the altar. These details shaped the Christian image of the seraphim as fiery beings closest to God.
The familiar nine-order Christian hierarchy was systematized by Pseudo-Dionysius in the late fifth or early sixth century. In that scheme, seraphim form the highest choir, followed by cherubim and thrones. Dominions belong to the second, middle group, so they are not the highest order.
Cherubim are a common mix-up because they are also exalted heavenly beings. However, traditional Christian angelology ranks them second, and biblical descriptions of cherubim differ from Isaiah’s six-winged seraphim.