For a more medieval weapon? A heavy-bladed greatsword, with a long, curved, and serrated blade. It wouldn't be very good for normal slicing or cuts, but it would be extremely painful to whoever you use it to tear into. Not only that, but it should also avoid the problems that normal blades have to contend with, such as chipping and dulling of the blade itself.
But for something modern? Either an automatic, belt-fed shotgun, or a lightweight, bullpup, scout sniper rifle. Either one would actually suit me, because I tend to either want to be sneaky at long range while keeping mobile, or right in the enemies face, unleashing an unstoppable cloud of buckshot in my path.