any CC would be Primary coverage if the renter doesnt have their own ins. Im not the legal owner of the car I drive (borrow) thusly whenever I rent as long as that CC has car ins its Primary in my case.

however someone who does own a car it would be best to use a CC that offers Primary so that they wont have to report the damage to their own ins company 1st, as most CCs offer secondary ins that is you 1st need to report it to your ins and then the CC might pickup anything that wasnt already