When it comes to boxing, kangaroos, would take the gold. Male kangaroos often "box" amongst each other, playfully, for dominance, or in competition for mates. The dexterity of their forepaws is utilised in both punching and grappling with the foe. But the real danger lies in a serious kick with the hindleg. The sharpened toenails can disembowel an opponent. This is one animal with which you wouldn't want to find yourself stuck in a ring. However, kangaroos very seldom attack humans unprovoked.
Here's a clip of two kangaroos "boxing":