As the title states, all of the standard main marine weapons are good.
The m39, the m41, the l42, and the m37.
I see rants about how such and such gun sucks. Or this person never uses this gun or dumb shit like that.They are all very effective and highly capable of killing xenos. The issue I see arising is poor application of a good weapon. Not to the tune of "you just fucking suck try not sucking and it will work"
(ok actually that applies too), but rather tactical application of the weapon.
m39 shreds and moves quick as fuck wielded. m41 shreds and has pretty good accuracy. l42 with qfa shreds, and with smartscope ive used it to battle buddy with high success. m37 pbs are king of close range damage, high risk but high reward.
Some guns outshine others in certain roles, no one gun is king of all purposes. The guns arent bad some of you guys just expect them to do retarded shit.