Honorable mentions: galarian darmanitan, cinderace, ash-greninja, dragapult, and probably single-strike urshifu. Gengar may be a round goofy ghost, but there is a reason why it is in the same Smogon tier as Ultra Necrozma and Primal Groudon Obviously, you can switch out with U-turn or Volt Switch, but it already lets Gengar get to +2 SpA for free, a potentially game-winning move. Mega Gengar is so broken, that even HAVING a defensive/stall pokemon on your team probably means you have lost before the match even started. Another Hoenn option, Torchic first morphs into Combusken, then the fierce Blaziken, kicking off a long tradition of fire/fighting starters. Generation/Games: 3/Ruby, Sapphire, and Emerald. Mega Gengar can literally switch in, traps it with shadow tag, and set up 3 Nasty Plots in their face. Torchic, Combusken, Blaziken, and Mega Blaziken. Shadow tag alone completely invalidates any defensive pokemon that relies on Toxic or Seismic Toss for damage. Boasting the special attack stat of Kyogre, the speed of Tapu Koko, it will do well on its own, but Mega Gengar also gets the ridiculously overpowered ability in shadow tag. Serious answer: in terms of national dex singles, I would say Mega Gengar qualifies as the most strong and unfair non-legendary non-mythical pokemon.
