1) My wife.
2) Black hole of amusement. (For instance, building a home-made catapult and having fun hurling dog droppings into the yard across the street. This person would instantly make that unfunny and make you stop.)
3) A person so anti-funny that if you mixed humor and this person together, the resulting explosion would rip away Earth's atmosphere and make it an unliveable planet for thousands of years.