before all the gen 6 changes I probably would have guessed dragon or steel, but now it seems like that might be the case. Fairy seems reasonably strong but not unbeatable, I'd say for raw damage fighting is pretty amazing. Other than that Ghost is fantastic.
water or fairy. water's only real weakness is electric since grass isnt usually too offensive and most water pokemon leanr ice moves anyway and both pf fairy's weaknesses are very rarely offensive type moves bar scizors bullet punch