The best LMG in Call of Duty: WW2 must be the best at holding down a position against many incoming enemies. The MG 15 is a good candidate as it has excellent iron sights, fires quickly which makes it harder for enemies to fire back and has controllable recoil. When you are locked down in bipod mode in an advantageous position it will be hard for enemies to kill you.
However, the MG 15 runs out of ammo quickly due to its high fire rate. The MG 42 with extended mags has 100 rounds between reloads and shares the favorable characteristics with the MG 15 making it excellent at holding off waves of enemies. Therefore, the MG 42 is the best LMG in Call of Duty: WW2.