Best Hog killing gun/caliber? Whatever is at hand.
I'd go one step further and say whatever you have on hand that you can shoot accurately. The less the killing power, the more accurate it needs to be. For instance, even a .22 Hornet is instant death on even the biggest hog IF you can place the shot behind the ear. That same caliber in a heart/lung shot is inadequate for anything but very small hogs. .223 is a little better, but you still need precise placement to get that ear shot. Moving up to more power, allows a little more leeway.