My mistake; I completely forgot that Spiritomb already has a crest when coming up with my idea . Anyway, I came up with a couple other suggestions that I think will be suitable crests. If I do repeat an idea that's already been mentioned here, than I apologise.
Aurorus Crest: When it enters the field, it automatically sets up Hail and Aurora Veil.
Tyrantrum Crest: When it uses a move that's powered up by Strong Jaw, recovers HP equal to 1/6 of the damage dealt.
Masquerain Crest: Grants an immunity to Rock Type moves, including damage from Stealth Rock.
Purugly Crest: When it enters the field, the opponents Attack OR Special Attack is lowered by one stage, depending on which stat is higher.
Floatzel Crest: Protects it from Grass type moves and boosts its Attack by one stage (much like Sap Sipper)