I can't pick as it depends on what I'm shooting, I learned to never underestimate the power of a Shotgun as they're awesome at killing everything in one hit, especially flood! It defo makes a mission easier. Energy Swords and Binary Rifles are awesome too in this respect but the only downfall is the low ammo and the constant need to replenish or replace them.
That said if I had to pick I'd go with the good old reliable Assault Rifle. Pretty much a good balance in all the feats required of it.