A chainsaw, in any game that has them. Either used against me or as a weapon against the foe's that come at me. There is just something visceral about a chainsaw and they make the coolest noises when used on body parts.
I'm going to go with the Aliens from the "Alien" series of movies. If you go into the science of them they are one of the most perfect killing species in the cosmos. Acid for blood, sneaky as hell, and vicious!
I have always loved Star Wars since I've been a little kid. The universe has had its low points (the most recent movies being a very low point). But through it all I can still find the magic and wonderment every time I watch or read Star Wars.
I'm glad to see this trend of giving us (the players) consequences for our actions. I'm a huge Monkey Island fan but it was always weird to me that you couldn't die. Being a fan of sierra games where you could it was sometimes a difficult transition. Over all having our choices matter makes for better players and imho people. Maybe its the old school gamer in me, but not having a fear of dying, in games, makes them boring.