Surely it must be the King of Beasts — the lion, no?

Water bears, right?

Considering their resistance to extreme temperatures and environmental adaptability.

Humans nanodesu.

They can kill any beast just fine with guns. And for critters with tough hides or shells they could just use poison.

Huh? Humans are weak against bacteria and other microbes, you say?

Meh, just wear a Hazmat suit then and flamethrower everything into a fine scattered ash. Nanodesu.