What everybody else above said is correct.
The United States, Canada, and the other countries mentioned should be at the top of your list.
But if I were you, I'd go beyond a simple country filtering and focus more on activity levels.
I would focus on those four countries, but I would go deeper.
I would focus on which type of groups has a lot of activity.
I would also cross-reference these groups' topics with affiliate offers or other forms of monetization.