To some, a "church father" is a commentator (ancient or otherwise) whose commentaries they prefer. To others, the appellation "church father" defines those (orthodox or not) who lived within a certain era. To still others, a "church father" is one who reflects, to a greater degree, the church's "tradition" (depending, subsequently, on one's understanding of "tradition").
In Ambigua 7, St Maximos the Confessor offers what I find to be one of the better definitions of "church father." He designates them as "our fathers who are wise from hearing the Scriptures read in the divine mysteries."
This definition does not suffer from the limtations of "favorites" (favorite era, favorite comments, favorite theology). Rather, it places our definition squarely within the locus of tradition; namely, the Divine Liturgy or Holy Mass, where dogma is lived in prayer. St Maximos' definition also has the advantage of reminding us that piety by attention to the Divine Liturgy is the distinguishing mark of a saint--which a "church father" should be.