Users are allowed to view all data within our Org, with one exception. Users in Region X are not allow to view leads from other regions but then can view any leads within their region. How do I set this up?
In response to your comment:
No. As you go along the different levels of security you can only widen access, you cannot restrict it. OWD is the first level, if you have Lead as public/rw at that level everyone can rw Leads regardless of role hierarchy or anything else.
Set the OWD to be private and go from there.