Can someone explain to me who some of the most significant Fathers in the Armenian Apostolic tradition are with respect to doctrinal definition?
Is Saint Sahag of great importance in this realm?
If you mean St Sahak Parthev who translated the Bible, then we have only some dogmatic letters from him (the other extant writings of his are a group of canons in our Nomocanon and some hymns attributed to him). Frankly speaking, I don't fully understand what you mean by "with respect to doctrinal definition". As all notable Fathers of our Church who have left literature that deal with this or that aspect of the general teaching of the Armenian Church, that is, not only Christology, but also other branches of theology, are significant, and it is very difficult to say who is the most significant of them.
I think, first of all, the writings attributed to St Gregory the Illuminator are important, his homilies and the Catechetical writing called "Teaching", but the scholars say they were not written by him, but most probably by St Mesrop Mashtots. The language and the ideas of those writings predate the Council of Ephesus. From the fifth century's Fathers St John Mandakuni is an important father, also Eznik of Kolb for his treatise "Te Deo" or "refutation of the sects": this work deals mainly with the issue of good and evil and is written against dualistic teachings of his time. It's a good source of information about some heresies, of Marcion, Mani etc.
St John of Odzun (VII-VIII centuries) is a great father. Some consider him to be the final stage of the formation of the Armenian Church Christology through the teaching of incorruptibility. From that period, Khosrovik's dogmatic writings are very important, John of Mayravan too. Stepanos of Siunik is a notable father. Vrtanes Kertol, like St John of Odzun and John of Mayragom, defended the veneration of icons, so he is important for that. From later Fathers are notable John the Deacon, Paul of Taron, John of Erznka, Vardan the Eastener (Vardan Areveltsi), St Nerses the Grace-filled, Nerses of Lambron, Mkhitar Gosh, St John of Orotan (the teacher of St Gregory of Tathev), Mkhitar of Skevra (this father is more known for his treatise “On the 12 Apostles' equality in honour”) etc etc.
St Gregory of Tathev is perhaps the last great vardapet (that is, "teacher") of our Church. He tried to systematize the teachings of the Armenian Church. But in his sacramentology he was influenced by Aquinas, the scholars say. However, he remains a father to whom first of all we turn when trying to find answers to this or that thing. But there are also some of his pupils, Fathers of the Tathev school, that are notable too, for example, Matthew of Julfa and Arakel of Siunik.
Well, it is really very difficult to say who of them is more significant, each may be significant in some aspect. And most of them are not fully examined, not all of their writings were even published. So they may be not well known because of this. I can say only that more often the names of St Nerses the Grace-filled and St Gregory of Tathev are heard only because they were more popularized. But the Christological writings of, say, St John of Odzun, or John the Deacon or Paul of Taron are much more mature and important writings than those written by the aforementioned well-known two Fathers.