Most of the class units for the theocrat benefit from the increased melee strength of the orcs as well.
The orcs strengths work well for Theocrat as well, all their units having more health makes martyrs much more useful(more on this later) and Orc crusaders are the strongest variant of crusaders in the game(they begin with the tireless ability). On one hand the Theocrats units and spells help cover the orcs primary weakness, low resisitance, (one pretty early theocrat spell gives a unit +6 resistance for two turns). Orc Theocrat is probably the strongest combination for Orcs for several reasons.
These are all units that can be given the devout property by the theocrat, which is important for getting out lots of damage. In addition to class units that alter by race, you also wat to consider three primary unit types that the theocrat focuses on, namely cavalry, infantry, and supports.