The Thompson is probally my favorite too..close second is the German Sturmgewehr 44 (StG44).
The Sturmgewher 44 was the world's first true assault rifle and was introduced by the German army late in WWII.
It was the direct inspiration for the Russian AK47, the most prolific gun in the world.
If the war had continued another year, the SG44 would have replaced every other rifle and submachine gun in the Wehrmacht.
The assault rifle proved an invaluable weapon, especially on the Eastern front, where it was first deployed.