If you are in one country, it is straightforward: choose that one.
If you are spread across multiple countries and offices, the choice you need to make is based on where the majority of your employees are based on a Full-Time Equivalent basis. Not where you generate the most revenue or anything else.
As such, it may mean you don't choose where your headquarters are or where you are legally registered.
It may mean you have to do a full analysis of your people by full time, part-time and temporary workers.