The German helmet was the best during WW2 because it knew what it was doing, and it did it well.
The role of a helmet for a soldier is not to block bullets or directly firing projectiles, but to instead protect the head from artillery and bomb shrapnel. In this sense the Stahlhelm was the best as it got full coverage.
The entire head is protected, and the skirt around the top makes sure that anything which hits the top rolls off harmlessly. It also slightly protected the back of the head with the little skirt which dropped down a little behind.
The design was by no means perfect though, the steel used to make them at the beginning was high quality, but as the war raged on the steel would drop in quality. During WW2 however this was easily the best, the second best probably being the M1 and their similar counterparts.
