Flame thrower.

Agree, the cost to make one was incredible compared to others.

Ive shot a M1 Garland several times. They are some heavy suckers.

Probably the M1 Carbine due to the weight.
That said, I would really liked to have inherited my father-in-laws Presentation Grade Thompson.

Guberment was looking for a low cost alternate to the Thompson which was very expensive to make.

The only "safety" on the M3 was the dust cover...interesting.

I carried an M60 with a 1911 as my sidearm in the Army. After 7 years of lugging it around, I'd say M1 Carbine or M3 grease gun. Followed up by M1 Garand, Thompson, BAR, and in last place a M1903A4. I own or shot them all. While I love the M1903A4, there's something to be said for volume of fire. Folks who haven't the joy of neck and shoulder pain of carrying a heavy weapon and ammo for long distances overestimate their abilities to do so.

Machine gun and mortar teams I'll bet had to lug stuff around

Knowing what I know now, an M2 carbine.

My Dad was a Marine Recon Sgt in Korea ('52 - '53). Dad carried a Thompson with the rear stock removed.
One guys in his squad carried a Grease gun until the night they were behind enemy lines and the guy slipped down a hill and it started going off because of the 'safety'. Dad banned the grease gun from his squad after that.