Best Hog killing gun/caliber? Whatever is at hand. If I thought shooting one in the ass with a pellet rifle would cause it to get gangrene and die, that'd be enough of an arsenal for me at tha time.
My favorite is still my .243 with a 70 or 90 grn Ballistic Tip to the ear hole. We shot one the other evening with a .308 and 165grn BT to the neck, that made a hole on a 250+ lb sow who was about to spawn more vermin.