The best submachine gun in Call of Duty: WW2 has got to be extremely good at close range and competitive with assault rifles at medium range. The grease gun is a lot of fun to use and very strong at close range as it can be fired from the hip and still have good accuracy.
However, at medium range it is difficult to compete with rifles, especially if enemies are behind cover. The submachine gun that is even better than the Grease Gun and can compete with rifles at medium range is the MP-40. It can be fired whilst aiming down the sights and compete with rifles at medium range and also has strong hipfire characteristics for close range. That means it is best suited to the role of submachine gun which is to be close to enemies where they don’t expect you to be. Therefore, the MP-40 is the best submachine gun in Call of Duty: WW2.