For a modern theologian, I'd recommend Metropolitan John Zizioulas, specificially his Book 'Being as Communion,' it's an interesting and good approach to theology focusing on the importance of Eucharistic Theology as central to the Life of the Church and equating it with Ontology. Furthermore, it's a relatively easy read (for Orthodox Theology)
If you're looking for Patristic sources, you have to read Pseudo-Dionysius the Areopagite's 'Mystical Theology,' all of his works are worth the time to read, but the 'Mystical Theology' is only about a dozen pages or so and fundamental to the Orthodox Apophatic approach to Theology. If you want a harder, but equally rewarding, read, there's St. Clement of Alexandria, I love all of his stuff. That's all I can really think of off the top of my head for Fundamental Theology (St. Maximos the Confessor had a few works, but I can't remember the names of the ones that fit in this category). Of course, if you want to get into Christology, Trinitarian Theology, Soteriology and all the other Specifics, then there's lost out there: I'd start with St. Athanasios, St. Gregory of Nyssa, St. Gregory the Theologian, and St. Basil the Great. For Moral Theology and Ethics, you can't beat St. John Chrysostom...it really depends on what exactly you want to read, there's a lot out there. If you ever find an interest in Canon Law, then I can give you a real bibliography
