M-M-M-Monster-NinjaGanthrinor said:MG42.
It's a really, really, really angry-sounding gun. I love it. Soooooo much.
High-calibre sniper rifles like the Barrett .50 come in a very close second. Bonus points if you can get them both going one after the other. The growling roar of the MG42 followed by a moment of silence then the head-shattering thunder of a high-calibre armor-piercing round breaking the sound barrier mere feet away to ruin somebody's day downrange.
Also, very well said.
OT:
I second this good sir, the most mighty gun sounds in my opinion.