Thompson is my favorite but it depends on the situation for best imo. I do like my mg42 heroic the "devils piano" though in a close second place runner up lol.
I've become partial to the new volksturmgewehr, its incredible fire rate along with it's rifle power. I feel comfortable in any situation. The type 100 was my fav., but at certain ranges it can get a bit dicey.
My personal favorite two guns in call of duty WWII are two guns that were added more recently one is the volkssturmgewehr rifle with FMJ and Advanced rifling in the resistance division and the combat knife with the hunkered and a Semtex. Another favorite gun of mine is the newest of the SMG's; the M-38 with FMJ and advanced rifling with hunkered and a rocket launcher with a throwing knife. I like these two guns because compared to the other guns these two guns have little to no recoil and with FMJ it does a nice increase to damage and advanced rifling because you never know how far away that damn enemy will be whether he is point blank or across the map.
Honestly in my opinion it depends on the map... like for lewis and mg-15 are good on gustav, uss texas, and ardennes forest. type 100, ppsh, and grease gun are good for pointe du hoc, flak tower, and london docks. The stg-44, bar, and fg 42 and good for santie marie, gibralter, and valkrie