I could go with St. Isidore.
Not sure about the good Dr.'s suggestion of Tomas de Torquemada - might offend some - kind of a sore point. ;-)
Saint Paul .... when we're dealing with the fundies, Calvinists, and other trying protestants.
Saint John of the Cross ..... when we need perseverance as Catholics
Blessed Padre Pio ..... when we need to engage the sins of the secular age and present the changeless truth of Christ and his Church.