So many wonderful, wonderful firearms to choose from...
One of my faves is Abigail Black's Sniper's Rifle with attached noob tube. If you miss the head, use the noob tube. (Game: Jericho)
Though another two rifles I like are the Hunting Rifle and the Anti-material rifle from Fallout: New Vegas. One shot from either will put a man down - hard. And one good shot from the latter will put a Super-mutant down.
Though when it comes to rapid fire mayhem, it has to be the FG42. She kicks like a mule, but she's a looker!