The most important are the Four Great Doctors of the East and West.
The Four Great Eastern Doctors:
St. Athanasius the Great
St. Basil the Great
St. Gregory the Theologian
St. John Chrysostom
The Four Great Western Doctors:
St. Ambrose of Milan
St. Augustine of Hippo
St. Jerome
St. Pope Gregory the Great
These 8 are extremely important.
Then there are the great Medieval Doctors:
St. Thomas Aquinas
Blessed John Dun Scotus
St. Anselm of Canterbury
St. Bonaventure
St. Bernard
Also there are the great Early Church Fathers/Apostolic Fathers:
St. Justin Martyr
St. Pope Clement I
St. Iranaeus
St. Polycarp
St. Ignatius of Antioch
^These are some of the Apostolic Fathers
Then other important Early Fathers:
St. Pope Leo the Great
St. Isidore of Seville
St. Maximus the Confessor
Finally, Church Fathers who are not Saints for various reasons:
Clement of Alexandria (although some Eastern Christians do venerate him)
Tertullian
Origen
Eusebius