Your gun only has one shot (round) you can fire. Which gun/caliber/loading are you going to choose, and why?
.45 caliber hollowpoint from my H&K USP Compact. Accurate enough to be aimed, and can put a man down even with the standard US GI ball ammunition. I would aim for just below the sternum, center of mass for maximum bodily damage with rapid expansion from the hollowpoint.
Now you've got two rounds and another choice of gun and ammo configuration? What are your choices now?
.45 caliber hollowpoint, this time I would double tap in the area previously mentioned.