In which county do I file a will?

The probate of the estate of a person who passes with a Will or without a Will may be in the court in the county 1) where the decedent has a known place of residence; 2) if no place of residence, in the county in which the greater part of the decedent's real estate is located; or 3) if no residence and no real estate, in the county in which the greater part of the decedent's personal estate is located. (See the Illinois Probate Act Section 5-1).  It sounds to me, with the few facts you have given, including the relatively short time in County B, that county A is the proper county. You will need an attorney to represent the estate anyway so I suggest that you contact an attorney that has a good reputation in your area and who handles probate estates and confirm which county is the proper venue. This answer is not intended and does not create an attorney/client relationship. The answer is just a statement of the general law and may or may not be applicable to your situation, depending what additional facts might exist. You should always consult an attorney and get specific legal advice.