Most important tank for WW2-era Germany war effort.

At first thought my answer was and still is the PzKpfw IV. I wondered how well the campaign in France in 1940 had gone without the Czechoslovakian tanks. But if they made the difference it was because of their numbers, not the types themselves.

 

PZIV gets my vote.

It was a great tank at the start of the war and was still quite formidable at the end.

 

I voted Panzer IV as a reflex of my long forum history, but I do think a better case can be made for the Panzer III. For the German war effort, no tank was more important than the one that carried it through the major campaigns of the early years; after that, it was a matter of delaying the inevitable, and even though the Panzer IV and V served admirably in that role, that doesn't make them spectacularly important compared to the humble Panzer III.