Just posting the ones I've read so far that I think are important, in no particular order:
1st and 2nd Apology, Dialogue with Trypho, On the Resurrection, Discourse to the Greeks, Address to the Greeks - St. Justin Martyr
Against Heresies - St. Irenaeus
On First Principal, Against Celsus - Origen
Stromata - Clement of Alexandria
On the Apostolic Tradition - Hippolytus
The Divine Institutes - Lactantius
Confessions, City of God, On the Trinity - St. Augustine
On the Priesthood, Homilies on the Gospel of Matthew, Homilies on the Gospel of John - St. John Chrysostom
The Sacred Writings of St. Jerome, Commentary on Daniel - St. Jerome
On God and Christ, The Five Theological Orations and Two Letters to Cledonius - St. Gregory of Nazianzus
On the Incarnation, Against the Heathens - St. Athanasius
On the Holy Spirit, On the Human Condition - St. Basil the Great
On the Trinity, On the Holy Spirit, On the Soul and Resurrection - St. Gregory of Nyssa
On the Mysteries and On Repentance, On the Holy Spirit - St. Ambrose
De Trinitae - St. Hilary of Poitiers
Book of Pastoral Rule - Pope St. Gregory the Great
On the Cosmic Mystery of Jesus Christ - St. Maximus the Confessor
On the Unity of Christ - St. Cyril of Alexandria
The Ladder of Divine Ascent - St. John Climacus
The Fount of Knowledge - St. John of Damascus