It's unusual for a U.S. credit-card holder to have other insurance which would cover loss of or damage to, a car rented overseas. (I think that Chubb offers an umbrella policy which covers this.) So, any U.S. card which offers free rental-car coverage -- even "secondary" coverage -- should be primary when renting overseas. Of course, there are differences from one card to another as to the details of that coverage: types of vehicles covered; length of rental covered; dollar limits; excluded items; excluded countries; etc., etc.

And for domestic rentals, the CSP and CSR cards offer primary LDW coverage (except for cardholders who reside in New York State).