Since I've never used a firearm*, I don't have a favorite. However, my father's (he served on PT boats in the Pacific) favorite automatic weapon of WW2 was the 40 mm Bofors**. He wasn't particularly fond of the Thomson SMG.
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* I neither like nor dislike guns, per se; my life just makes them totally superfluous, as I don't hunt and don't feel any particular need to "protect my home." I'm also not fond of loud noises.
** I wonder if he was thinking "go big or go home." He also got a hernia lifting the receiver out for maintenance.