Hah! The scariest Pokemon is definitely Shedinja. Here's a scenario:
You: Hmm, he's got a Venasaur...Hah! He can't hurt Shedinja! Go Shedinja!
Other Guy: Uh oh...
You: *You accidentally let your eyes wander and you look into the hole in Shedinja's back seeing as your behind him. You die. Game over.*
Shedinja's really scary! If I had him I'd just hold him like a steering wheel with his face looking at me, and make people look at the holw in his back, .