I'd say: defensively: Dragon / Steel weak to fighting and ground resistant to normal, water, flying, grass, electric, psychic, rock, bug, ghost, dark and steel (11 in total) immune to poison neutral to fire, ice and dragon
offensively: ice / electric because of STAB Boltbeam
Waiting for golden Sun 3DS 2 things are infinite: The Universe and human stupidity. I'm not sure about the Universe, though -Albert Einstein
It would be the best with Wonderguard, that's for sure. Without it, it isn't very good in terms of offensive STAB coverage (good v.s. Psychic, Ghost, and... nothing else) or resistances (Normal, Psychic, Fighting immune and Poison resist). Sure it may have no weaknesses but getting hit by normal effectiveness moves is going to hurt.
Defensively, I'd say Ghost/Normal with Levitate. Four immunities (because of Levitate) and a couple resistances. Plus, it's immune to a couple things that are normally SE against either Normal or Ghost (immune to Fighting and Ghost respectively). Its only weakness is Dark, and that's only 2x effective.
Another great combo defensively is Bug/Steel (done before, I know). Its only weakness is its 4x weakness to fire (why are you sending a Bug Type or Steel Type against a Fire Type anyway?) and it has a ton of resistances and an immunity to Poison.
Offensively, I'd say... Dragon/Fire. Perfect neutral coverage. Name a single Pokemon that resists both (Heatproof doesn't count as a resistance).