What is the best 30 amp shore power cord brand?
I think maybe "best" would be the new SmartPlug (https://smartplug.com) designs.
But as far as 'regular old' shorepower cords, I like most the ones which have an LED indicator on them that shows you at a glance whether or not they are live.
I am not aware of a quality difference in the relative brands, but there probably is one.
Whatever the make, I always keep an eye on the ends for loose fitting & corrosion, because that's where the fires start... :o