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The answer will always be Mega-Gengar. Were it not immediately banned to Ubers (where it was competing with fucking Primal Groudon for top-billing), it would've made choice items impossible to use, since it could just switch in on a Banded Close Combat or Specs Boomburst and just set up 3x Nasty Plots for an instant win scenario.
Luckily it was yeeted into the depths of ubers, but there are some really fucking toxic Pokemon that plagued earlier metas, like Cacturne in Gen 3 or Chansey in Gen 1.