None of them would have stopped a rifle round, because that wasn't what helmets were designed or expected to do. Infantry helmets are meant to deflect shrapnel and splinters that would glance off a thin steel helmet, but would cause a debilitating wound or death to a soldier with an unarmoured head. The only bullets a WW2 helmet has a hope of stopping are ricochets or a small-caliber bullet fired from a long distance.

On that note, was there a best helmet? Honestly, I'm sceptical that any one helmet was that much better than any of the others. German steel helmets were heavier and a little thicker than most others during the world wars, and after the war the stahlhelm shape and design has proved popular, including with the modern US army, as it covers more of the sides and back of the head and neck. Britain similarly re-designed its helmets in 1944 to provide more protection. But in the end all of them were fairly cheap and simple designs meant to provide soldiers with a slight bit of head protection.