Here are the number of citations of authorities found in Secunda pars in the Summa theologiae of St. Thomas Aquinas. Obviously, St. Augustine and Aristotle are the most prevalent.

Augustine 1,630
Aristotle 1,546
Gregory the Great 439
Pseudo-Dionysius 202
Cicero 187
Jerome 178
John Damascene 168
Ambrose 151
Isidore of Seville 120
Roman Law 102
Gregory of Nyssa (actually Nemesius of Ephesus) 41
Macrobius 33
Boethius 30
Prosper of Aquitaine 19
Benedict 18
Basil 13
Plato 12
Hilary of Poiteiers 12
Bernard 9
Caesar 8
Ptolemy 1

These are based on Busa’s concordance and lifted from Servais-Theodore Pinkaers, OP “The Sources of the Ethics of St. Thomas Aquinas” in The Ethics of Aquinas, Stephen J. Pope, ed. (Georgetown University Press: Washington DC, 2002), 17.