In my experience, a long, thin, rigid, flatground blade is the ultimate cardboard cutter. I have some hollow ground blades, including smallies, that do quite well. But if there is a heap of boxes to break down, I will take the Old Hickory Large Butcher Knife. It's thin, (very long / 1'2"), not exactly flat ground (rather a sabre grind) and made of excellently treated 1095 steel. It's one of the few knives I did resharpen only once because it still performs brilliantly after one year of use. Tldr version : long, thin, flatground, sharp... and preferably cheap because cardboard ruins the edge quickly. Sharpening will be frequent, whatever the steel, so the blade will diminish rather quickly, so initial cost is a factor.
So, to your question... if you really want/need a folder, the Opinel N°9 or 10 would be your best choice, IMO.