More noobish theorymoning:
Multitype Blissey, holding a Spooky Plate. Just for the fun of people trying to hit it with Fight moves. Before the trick is revealed (when somebody uses a fight move), nobody would even think about using a Ghost move on Blissey, while extremely few Pokémon carry a Dark move as their main anti-Blissey move, so the trick isn't likely to be revealed by accident.
Or you could go for any other plates that give a better defensive typing.
On the downside, you miss out on Leftovers...