Smogon's OU list by gen
Gen 1: Clefable, Ditto, Dugtrio, Gyarados
Gen 2: Tyranitar
Gen 3: Pelipper
Gen 4: Rotom-Heat, Rotom-Wash, Togekiss
Gen 5: Bisharp, Conkeldurr, Excadrill, Ferrothorn, Hydreigon, Mandibuzz, Seismitoad
Gen 6: Aegislash, Hawlucha.
Gen 7: Mimikyuu, Toxapex
Gen 8: Barraskewda, Cinderace, G-Corsola, Corviknight, G-Darmanitan, Dracovish, Dracozolt, Dragapult, Grimmsnarl, Toxtricity (Both Forms)