i have great affection and admiration for Athanasius. He fought a very difficult war against a powerful foe, Arian heresy. Today there's a group styling itself Arian Catholic Apostolic Church with "bishops" and "presbyters"
I am a super fan of the Capadocian Fathers, whose contribution to the life of the Church is enduring.
St. Hilary of Poitiers: http://newadvent.org/cathen/07349b.htm http://www.excitingholiness.org/first-edition/index.cgi?m01/d13a.html I only started reading this holy father in depth a couple of years ago and his insights into the Gospels and focus on the sacraments, especially the Eucharist, has been very enlightening to me. His focus is more direct than some of the other fathers and his writings are thus less obtuse.
All of the above mentioned saints are wonderful. Two of my favorites are St. John Chrysostom and St. Francis de Salles. I greatly appreciate their down-to-earth style of preaching/writing, their gentleness, and their emphasis on Christian love.
I've only read a handful of the great early church theologians but I would have to go for Athanasius - loved his book "On the Incarnation" and his story how to took a stand against Arianism is pretty stirring. Does Martin Luther count too? I admire much of his writings