I'll have to go with World War 1. The least useful of Germany's allies probably had to be Bulgaria, but I guess they had some limited use in finally rolling up Serbia. The Ottoman Empire and AH pretty much raced neck to neck for most useful allies. One on hand, the Austrians had a bigger army and economy, but on the other, their record in 1914 was appalling (though it improved somewhat in later years). The entry of the Ottomans pretty much cut off Russia from the rest of the world, and drew troops away from other fronts, requiring relatively little aid from Germany to do so (mostly monetary aid as well as some officers, rather than the huge troops numbers AH needed). Overall, I'd have to give most useful ally to the Ottomans.