If my father passed in one county but he lived in another county, which one county must I file for probate as an administrator to his estate?

I am so sorry for your loss.  You file the petition for probate and to be appointed as the administrator of the estate in the county in which he resdied at the time of his death.  His legal residence.  Now, it is my understanding that a person residing outside of Virginia who wishes to be appointed as administrator must bring a Virginia resident to the appointment with the Probate Clerk to either co-qualify or be designated as a registered agent. If you do not have a Virginian co-qualify as administrator there may be other requirements under Virginia Code 26-59 but I saw some writing that indicated that section is repealed after Ocitober 1, 2012.  So I would speak with an attorney there.  Good luck.