I remember German infantry during occupation of Norway in 1940 were estimated (by German historians) to be 2x combat effective compared to British infantry. I.e. 1 German infantryman fought 2 British on approximately equal grounds. Japanese infantry during late 1941 was 2.25x more effective compared to British (well, the mixture of British regular and colonial troops). Therefore, in 1940-1941 Japanese and German infantrymen may be the most capable.
As the time passed, official Japanese estimates by 1945 showed majority of infantry units being 30% combat efficient compared to 1941 standards (making them ~0.68x of British infantry of 1940-1941) due deficient training, equipment and supply.
I do not remember any comparison for other forces, but suspect US was comparable to British or slightly worse (from disparaging remarks by German regarding early US actions in Africa) initially. Soviets were considerably inferior to German or British in 1941.
During the war, the states with the lightest casualties rate (US, British) have improved most, while states with worst casualties (German, Japan) suffered the greatest loss in infantry combat efficiency. Soviet Union fall somewhat between - infantry efficiency improved over time, but many severe maladies arising from deficient training have persisted until the end of war.
Regarding Italy, it has clearly the worst fighting quality infantry. Italy had the same leadership problems as early British, but in much aggravated form. Leadership/command/tactics was so inferior what even superior Italian training and brain-washing (morale) was absolutely not enough to compensate for bad officers leadership.