If we're speaking about pure technical analysis it's hard to find primary sources and there is a lot of post-war speculation about various nations opinions on other nations equipment that I would not be too hasty to trust. The best lead would be to peruse this copy of the German published "Guide to Foreign Weapons" (Kennblätter fremden Gerats) which was issued to troops and commanders during the war.

I'm going to leave the technical side there because it seems difficult to find reliable information but there is another point I would like to make:

Throughout the war the small unit infantry tactics of different nations were generally different enough that the strengths of possibly technically superior weapon would be negated. I'll leave the objective comparison of German and American weapons to the frothing historical-gun-nuts who are wholly more qualified than I am to make such comparison. Rather I would posit that, for example, the German opinion on the semi-automatic M1 Garand rifle would have been that it was wholly unnecessary and expensive a tool for a rifleman to fulfill his role of assaulting positions and providing lethal fire at range. This is because German infantry tactics at the time called for suppression and elimination of the enemy to be performed by a squad central machine-gun while riflemen and grenadiers were there only to assault positions and take available shots from a position of immense fire superiority. For this role, a simple bolt-action rifle is more than sufficient and much much cheaper.

Contrast this with the American tactic of providing decentralized fire superiority from a squad of almost entirely riflemen and it become obvious why they placed such value in a mass-issued long range semi-automatic firearm. Rather than operating in a firefight with fire superiority assured, American riflemen had to fight to establish that fire superiority with his own weapon.

This article does a good job of explaining the differences and it corroborates with what I have read elsewhere. These differences in tactics generally dictated the development of small arms and so each nations arsenal generally corresponded best with how they fight. As a result, I find it hard to believe that any nations during the war would have so coveted the weapons of any other nation at any sort of tactical level based on their usefulness.

That being said, soldiers have and always will be soldiers, and if there is one thing soldiers love to do it's complain about their equipment. So no doubt pilfering of enemy or even friendly equipment happened on a smaller scale on all sides of the war.