The Holy Bible is of course the greatest work; I don’t want to pick it apart and say book X is better than book Y, because I prefer to view it as a unified whole, with material explaining the meaning of life, the basics of morality, Christological prophecy, and the Psalms, in the Old Testment, and then the glorious Gospel narrative in the New Testament, which is then reinforced by and expounded upon through the Acts and the Epistles, and finally, there is an eschatological thread going straight through the Bible culminating in the Apocalypse.
I think we can all agree that the Bible is the best of the ancient theological works.
However, the OP was, I expect, asking for a reccommendation of books on the subject of Orthodox theology which explain the interpretation of divine revelation, in the Bible and elsewhere, as held by the One Holy Catholic and Apostolic Church.
So, for my answer, among ancient works, I particularly like everything written by St. Athanasius, such as De Incarnatione, but I especially enjoy his biography of St. Anthony, and I also love On Heresies by St. Irenaeus, the Panarion of St. Epiphanius of Salamis, and the writings of St. (Psuedo) Dionysius the Aereopagite. Among modern works I think the Philokalia, Orthodox Dogmatic Theology by Protopresbyter Michael Pomazansky, the Orthodox Way by Metropolitan Kallistos Ware, the second edition of Orthodoxy and Heterodoxy by Fr. Andrew S. Damick, and in the grand tradition of OCNet, I highly reccommend Orthodoxy and the Religion of the Future by Fr. Seraphim Rose.
Of these works, Pseudo Dionysius demonstrates to us an apophatic way of thinking about God, Metropolitan Kallistos describes this technique in great detail in the Orthodox Way, and Against Heresies, the Panarion, Orthodoxy and the Religion of the Future, and Orthodoxy and Heterodoxy allow us to comprehend the Orthodox Christian faith itself using the Apophatic method, through a via negativa of comparisons against heterodox distortions of the Christian faith, and other religions such as Islam and Hinduism.
I hope that answer helps.