Ghost/Steel or possibly Steel/Poison w/Levitate (weak only to Fire, with resistances to everything else but Psychic and Fighting) are the best defensive type combos. However, offensively, nothing beats Fire/Dragon; the only thing that doesn't at least get neutral damage is Steel/(not Dragon) with Flash Fire/(Heatproof and Fire/Water/Rock).
Ghost/Normal is an interesting type combo. Walls a popular Arceus set, has 3 immunities and only Dark for a weakness, et cetera.
I'm going to go with either Fighting/Flying or Fire/Water, to finish up one of the sets, though; it has more obvious value to normal players than competitive stuff; who cares if it walls an Arceus set if you can't explain what EVs are.
For those of you starting topics about the PS4, there's a PS4 board: http://www.gamefaqs.com/boards/691087-playstation-4