Gengar

Thanks to the incredible ability Shadow Tag, Mega Gengar is the best trapper in the metagame. It has high Special Attack and Speed stats, allowing offensive sets to outspeed and revenge KO common offensive Pokemon such as Darkrai, Arceus, and Mega Rayquaza while stopping them from switching out. Gengar's large movepool includes powerful offensive options such as Shadow Ball and Focus Blast for perfect coverage alongside support moves such as Destiny Bond, and sets with Taunt or Perish Song destroy stall and balance, particularly passive Pokemon such as Clefable and support Arceus. It is incredibly effective at supporting sweepers by removing their answers, such as Arceus-Fairy for Darkrai. Mega Gengar also has an excellent offensive typing that allows it to take on common metagame picks such as Xerneas, Lugia, Arceus-Fairy, and Mega Diancie without being a slouch defensively either thanks to its immunity to Normal-type moves allowing it to revenge KO Arceus and Mega Rayquaza without worrying about Extreme Speed. However, Mega Gengar cannot be run alongside other Mega Pokemon such as Mega Rayquaza, making it much harder to fit onto a team. Its paltry bulk prevents it from taking any non-resisted hits from nearly any offensive Pokemon, and it has a decent number of solid checks, forcing it to play as a hit-and-run trapper