I play medic and their proficiency is with SMGs, So trying to stick with that.Some of the SMGs are ok, but there's some really, really good short range ARs like the M5 that just shred. LMGs can just spray tons of bullets too.
Most situations and ranges just favor other guns. SMGs can be good if you're defending an indoor point or something.
The PP29 is probably the best one. The PBX is one of those good but not great kind of guns that gets outclassed a tiny bit by everything else. Get outside its effective range (which isn't super far) and the thing just gets you hit markers.
I rarely pick SMGs, just based on how there's too much combat at even medium range.
The M5 is just kind of stupid good right now. You can set it up for the close combat options where the rate of fire goes up and outclass any SMG, then change it to a medium range setup and dominate there too. The M5 has pretty much been the best gun in the game since launch.
On paper the AK-74u seems the best, but Dice in their infinite wisdom made its unlock mission AR based