The STEN

By many accounts, the Soumi M31 was the best subgun of the war. Superior accuracy and reliability, and a huge drum magazine.

https://www.youtube.com/watch?v=LxHeCyqbHk8

 

The Suomi wasn't cheap though! On a side note, the Finns seem to have an excellent record with designing and producing high quality small arms. The Valmet is reportedly one of the best AK type rifles produced today.

On tanknet a Serb or Croat user who collects Yugoslav military manual said the Yugoslavs had a trial for most WWII submachine guns. The STEN was about 10% less accurate than the MP-40 when firing 5 round bursts at 100 meters. IIRC neither the STEN nor MP-40 stood out in hit probability at ranges up to and over 100 meters; the Thompson outperformed both but was in turn outshot by PPsh-41.

 

This chart gives some interesting data on 9mm from a 16" barrel.

The blue line (1350 fps) is probably accurate for a standard 9mm from that era, from a longer barrel. It's interesting that the trajectory brings it right back on target at 100 yards. I've found that true with a pistol as well - it may be a couple of inches lower from a shorter barrel, but it's still going to hit a torso sized target without holding high.

But look at the foot pounds of energy and how quickly they drop off! At the muzzle, the 9mm gives you 450-475 pounds of energy, but at 100 yards it's dropped to less than 300 pounds of energy. It's more of a wounding weapon than a killing weapon at that range.