As the resident mayor of Mario-land, I think Banette and Honchkrow are your big worries. Monarchy is interesting, but I would need to see proof. Ditto Crobat, which seems to have flopped. Toxicroak/Blissey could be very bad also if it caught on.
All these decks have their own problems:
(1) Banette - self-defeating, and weak to Absol/Honchkrow
(2) Honchkrow- Electivire is a big threat, as would be Ray EX d (if someone could find an effective way to play it in this format), and Raichu (still good)
(3) Monarchy- not sure if this can withstand Banette
(4) Crobat- ditto
(5) Toxicroak/Blissey- probably the best of the bunch, IMO.