Are we talking infantry weapon systems? I'll take it as if we are only speaking of personally carry-able weapon systems.
For flash or for practicality? For practicality, as a survival tool outside of war, Mosin Nagant rifle, solid performance, you can kill a man, a deer, or a bear with that gun with equal ease.
As for flash? Now that, is a tossup, as far as lighting someone up with a good dose of lead, the PPSH drum mag, was one hell of a hallway clearer, spitting a broad cone of deadly bullets at an insane fire rate, if your opponent doesn't see you coming, they're just straight up fucked before they even know what's going on, which makes it great for CQC and dense urban combat.
That or the MG42, that sucker was mean, sure you aren't going to be shooting from the hip with that thing, not unless you're a goddamn monster of a person, built like a brick shithouse, but if you set yourself up in a good spot with your field of fire covering a strategic point, the supressing/killing power of one of those machineguns, was something to be feared indeed...