Gourgeist. Easily. Just look at it:
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The small size's cry is freaking adorable. And it learns a move that involves taking the enemy trick-or-treating! Even if the thing is a sadist it's so cute!
In case you're wondering, its signature move Trick-or-Treat actually DOES take the opponent trick-or-treating according to Bulbapedia:
"The user takes the target trick-or-treating. This adds Ghost type to the target's type."
Garchomp.