KP m/44 would have to rate up there as it is the best refinement of the pps guns; and had 36 round stick mags or 71 rd drums. It also had a so so muzzle brake.
But most of us are in the US so who can not say the Thompson.
The Thompson had some of the best sights of any SMG of the era; better than many rifles even.
It is select fire, which is a huge deal if you really want to sight in a gun or conserve ammo.
It has a trigger disconnect for when the mag is empty that keeps the bolt from slamming forward without a cartridge to slow it down and more importantly to make it easier and faster to reload as all you have to do is replace the magazine. Most other SMGs of this time period need to be cocked after emptying a mag.
30rd sticks and 50 and 100 round drums put its capacity towards the top of SMGs of the time.
It fits a grown man not like some of the small stocks of many SMGs.
Its biggest faults are weight which made it more controllable and really only a pound or so more than most subguns of WW2 and the expense which should not be a factor for best SMG.
They were much better made than most SMGs of the war with nicely milled receivers and polished blued finish with nice wood furniture.
A mag dump with a 30 round Thompson mag will put a pound of lead down range in about 2 seconds much more than double what the mp40 will do.
The Cutts compensator was fairly effective for this time and most other SMGs did not even have a compensator.
45 ball > 9mm ball
And it is American!
With the M3 you have to stick your finger into the bolt to cock it.