Gengar

In a metagame like VGC 2019 where positioning is an essential aspect of the game, Mega Gengar is extremely powerful as its ability, Shadow Tag, gives its trainer superior control of the field by denying the opponent the ability to switch. Along with Shadow Tag, Mega Gengar boasts a huge base 170 Special Attack, which allows it to deal a huge chunk of damage on even neutral hits. Because of this, Mega Gengar can switch into Fairy-types and secure the KO regardless of how much health they have, since they are trapped by Shadow Tag. Despite its strengths, Mega Gengar is very weak defensively, which can easily lead to situations where it is threatened with an OHKO when the opponent has the Speed advantage. This can be played around with good use of Protect, Substitute, and Fake Out support from Pokemon like Incineroar. Mega Gengar also doesn't match up well against Primal Groudon as well as Tapu Lele, which often carries a Choice Scarf and can outspeed and OHKO it. Mega Gengar also doesn't like facing Dark-types like Incineroar, Yveltal, and Umbreon because of their access to Snarl, which hits through Substitute, as well as Ghost-types like Lunala, Mimikyu, and opposing Mega Gengar, which are all unaffected by Shadow Tag and can threaten Mega Gengar back.