D-Walker from Metal Gear Solid V: The Phantom Pain counts as a weapon, perhaps a vehicle as well, and he's also a friend! AND A ROBOT~!
He's so incredibly strong and tough and fast; in short, he's just perfect~ I don't think I could ask for anything more. In my game, he currently has a side-mounted minigun of some kind, probably an M134, two tranquiliser pistols, and a powerful arm that can pick people up and slam then into the ground, plus the arm also has a forearm-mounted machete!
Witness this display of Robotic battlefield dominance, with a flamethrower!
http://youtu.be/dirDhGP60to