From crablegs21 on Utreon:

"In your opinion and in hindsight, which country had the most optimized cartridge for the their primary infantry rifle for WWII? Additionally, which country would have benefited most by switching to that cartridge prior to WWII?"

In my opinion - and I think post-war development supports this - the best rifle cartridge was 8mm Kurz (7.92x33mm). The StG-44 was never actually Germany's primary infantry rifle, but they wanted it to be. The 8mm Kurz offers much reduced recoil ideal for semiauto and select-fire rifles, allowing fast followup shots while remaining powerful enough to be quite deadly.

However, this doesn't address the logistical issue of machine gun cartridges. Basically everyone in WW2 used a single cartridge for both rifles and support machine guns, and the cartridge choice was largely dictated by what was most effective in a machine gun intended to shoot out to 1000+ yards and to engage vehicles and aircraft. Larger cartridges were much better suited for this than today's intermediate cartridges. Interestingly, both the Italians (6.5mm Carcano) and the Swedes (6.5mm Swedish) developed supplemental big machine gun cartridges because they considered their standard rounds insufficient (these were 8x59mm for the Italians and 8x63mm for the Swedes)

If I had to pick a larger cartridge to allow both rifle and MG use, I would pick 7.35mm Carcano. This is about the lightest "rifle" round of the war that used a spitzer bullet and a rimless case. Its larger bullet diameter compared to 6.5mm rounds would allow better tracer and AP bullets, which are relevant in a military context.

As for which nation could benefit the most from adopting this cartridge before the war, I will go out on a limb and say the US, at least form a rifleman's perspective. The M1 Garand chambered for 7.35mm Carcano would be a fantastic rifle, probably at least a pound lighter than normal and faster shooting. The BAR could be lightened much more, and might have been able to be a more truly effective automatic rifle.