My list would add these to the others already mentioned:
Evagrios Ponticos
St. Macarius the Great
Archimandrite Sophrony (Sakharov)
Nicolaos Boulgaris
Nicolas Arseniev
George Fedotov
St. Peter Mohila
Patriarch Dositheus of Jerusalem
Apostolos Makrakis
New-Martyr Pavel Florensky
Vladimir Soloviev
Alexei Khomiakov
Nicolai Berdyaev
Ivan Kireevsky
I would note however that Apostolos Makrakis’ writings are at times extremely unorthodox (and condemned as such by the Greek Orthodox Church), though he was quite influential and certainly an important theologian. Florensky can be dubious in places (as can Bulgakov), Fr. Seraphim Rose is not without controversy in the Orthodox world, and most of what you’ll find in Soloviev - a strange combination of Sophiology, Gnosticism and Hegelianism which he tries to present as completely reconciliable with Orthodox Christianity - is dubious at best.
Regarding systematic theology, Orthodoxy does not regard theology as a scientific discipline whose main method is reason (taking the words of Scripture or the Fathers as data to be philosophically analyzed), as you might find in St. Thomas Aquinas. In this sense, there is no “systematic theology” in the normal sense of the term or even any “theology” in the sense Aquinas uses it. Instead, theology is regarded as the living experience of God - an experience seen through what St. Basil calls “the eye of the heart” or the nous rather than the dialectical reason. Consequently, to be a theologian in the most proper sense one should be a saint - as Evagrios Ponticos said, “the one who prays is a theologian, and the theologian is the one who prays”. It is possible to explicate the truths of the Faith as experienced and lived noetically in a quasi-“systematic” fashion, and indeed the magnum opus of Fr. Dumitru Staniloae - translated in English under the title “The Experience of God” - in the original Romanian was called “Systematic Theology”.
There are also catechisms (a method borrowed from the West) by a number of Orthodox theologians. The best ones are “The Divine and Sacred Catechism” by Apostolos Makrakis (which does have a couple errors in it) and my favorite one, “The Holy Catechism, or Explanation of the Divine and Holy Liturgy” by Nicolas Bulgaris (also spelled Nicolaos Boulgaris).
My strong recommendation is to start with the following:
The Experience of God, by Dumitru Staniloae
and then
Russian Piety, by Nicolas Arseniev
The Vision of God, by Vladimir Lossky
We Shall See Him As He Is, by Archimandrite Sophrony (Sakharov)
Nihilism, by Hieromonk Seraphim (Rose)
The Holy Catechism, by Nicolaos Bulgaris
Iconostasis, by Pavel Florensky