I don't think this question is specific enough. Do you mean the most produced and used small arm? Probably (Mosin Nagant). The most innovative and influential after the war? (STG 44). The easiest to produce? I would put that to the (grease gun).

The guns were produced based on the infantry doctrines of each of the military involved. The German forces were MG based squads so their whole tactic set focused on how to compliment that gun. Part of the reason the bolt action 98K was used for so long by the Germans was this focus on MG based tactics.

My long winded point is, that outside of personal preference, the question is probably not answerable in a pure factual sense. A close question would be what infantry units were the most effective with small arms. I would throw my money in for the Germans in 1941 as some of the most "effective" soldiers of the war. That is easy to debate though, because all of the major powers during the war became well versed in infantry based combat by 1945.

Such a loaded question, so many possible answers depending on how you clarify the question, so I'm gonna chime in with my own generic views.

Soviet Russia, on the Eastern Front with the submachinegun.

More specifically, the venerable PPS43 With around 2 million made during the war, plus the millions of PPSH submachine guns on hand, the Soviet Union with their easily deployed and rapid firing submachine guns, firing the powerful 7.62x25mm Tokarev round, managed to place massive firepower into the hands of millions of common soldiers.

The PPS43 was a marvel of simplicity (I regret selling my legal, semi auto handgun version of it, everyone needs a handgun with a 35 round magazine full of hyper velocity bullets that can defeat soft body armor, if nothing else for the cool factor when hanging on the wall). Made primarily of stamped steel, it could be quickly built, and most importantly quickly issued. The top folding stock allowed the weapon to be fired with the stock folded or unfolded, and the rudimentary sights extended out to several hundred meters. During the desperate rush to build small arms, the PPS43 could be made faster than any other weapon, and deliver more firepower, in a more compact form, than any other small arm in the Soviet arsenal. Excellent for house to house, and close quarters fighting, the PPS43 excelled in the hands of both the Soviets and the Germans, who found their 7.63mm Mauser ammo would also function in these captured arms. It is worth noting, that the famous picture of a Soviet soldier raising a flag over the Reichstag, shows a soldier armed with a PPS43 which would have been very useful in the close fighting within Berlin.

So, Tl;Dr, given no further qualifying information, I lean towards the submachine gun, specifically that in the hands of the USSR, given it's widespread use in some of the fiercest, close quarters fighting in the entire war.

On aspect not touched on here so far is the reliability of the standard small arms/service rifle. I own several Mosin/Nagant 91/30s, owned a k98 and have fired both the Arisaka and the M1. By far, the best made, easiest to fire and maintain was the k98. The Mosins, while extremely rugged, were machined poorly (at least all that I have fired), difficult to load and required constant cleaning to keep the bore from rusting out from the corrosive primers used.