Pokémon X and Pokémon Y

Actually Psychic types were the game breakers in Gen I, even more than Dragon Types, especially the fact that Special stat is hybrid of Attack stat and Defense stat except it's "special" oriented instead of physical oriented. There are no types that resist Psychic type attack other than the Pokemon of the same type, and its only weakness is Bug. I understand Ghost types are super effectives against Psychic, but that's not the correct statement in Gen I because the only Ghost type move that does actual damage such as Lick is dead useless against Psychic since it's immune to Ghost type moves. It probably would have been the same for Dragon types since it's special oriented if there were a Dragon type move other than Dragon Rage, but still.

[MENTION=1368]Baklava[/MENTION] That would be broken IMO. The best example is a Pokemon that is Dark/Ghost type. The only way to beat a Pokemon that has no weakness with a Wonder Guard ability is according to bulbapedia...

Hackers often hack this Ability onto Spiritomb (Wondertomb) and Sableye (Wondereye), because those Pokémon do not have a weakness. Hence, they can be damaged only by status ailments, weather conditions, entry hazards, Struggle, recoil, Life Orb, Sticky Barb, Black Sludge, Rough Skin, Iron Barbs, fixed-damage attacks, after the use of Gastro Acid, Entrainment (assuming that the user doesn't have Wonder Guard,) Worry Seed, Simple Beam, Foresight, Odor Sleuth or Soak, and by Pokémon with Mold Breaker, Turboblaze, Teravolt, or Scrappy.

Fire Fang, Future Sight, Doom Desire, and Beat Up work against Wonder Guard, but that was before Generation V, so that statement is irrelevant.