This is such a difficult, perhaps impossible, question to answer. It all depends on one's criteria. And if we are restricted to theology written in English or translated into English, as I am, then we are at a significant disadvantage! Surely there are many Orthodox theologians who have not yet been translated into English, who may well be unknown not only to non-Orthodox Christians but also to English-speaking Orthodox Christians. I had no idea, for example, that St Justin Popovich had written a dogmatics of the Orthodox faith. I'm sure there are many Orthodox theologians, whether Greek or Romanian or Russian or Serbian or whatever, whose writings are known only by their original audiences.
What Orthodox theologians of the 20th century can compare to the great Protestant and Catholic theologians of the 20th century?
Protestantism's great theologian of the 20th century was Karl Barth. Hands down. Catholicism's great theologians of the 20th century were Karl Rahner and Hans Urs van Balthasar. Hands down. They were great because of their erudition, creativity, produndity, and grasp of their respective theological traditions.
Are there any Orthodox theologians who can compare?
The only one who might compete with these three theologians is Dumitru Staniloe. I propose him not because I am well acquainted with his writings but because this is what other theologians say. Most of his writings have yet to be translated into English, unfortunately. Met John Ziziloulas is often cited here; but his writings are occasional, and he has become a controversial figure, especially among those Orthodox who have never read this writings. Vladimir Lossky was an important figure, but his writings are flawed by his anit-Western polemicism. He had an ax to grind.
Georges Florovosky was a great theologian, but, like Zizioulas, his writings were occasional. He never attempted a full-scale exposition of the Orthodox faith. But he was a great theologian. He understood that Orthodox theology could not remain in the ethnic ghetto.
I love the writings of Fr Alexander Schmemann and Met Kallistos Ware; but they are not in the same league as Barth or Balthasar. And as far as Fr Seraphim Rose, well, he ranks up there with Francis Schaeffer.