I would say Augustine is the most important for the western church, followed by Tertullian, and Athanasius for the eastern church, followed by Theodoret, Cyril, Basil, Maximus, Gregory, and Damascene.
How important a father is is very geographical. The western church read far more of Augustine than they read of anyone from the east, or even all of them combined, and the eastern church didn't read him much at all, but read a lot of the Greek fathers.