Xerneas

Xerneas's high stats across the board and good offensive and defensive typing allow it to run an effective offensive Poison Heal set, giving it added longevity to set up and damage the opposing team. If it is able to find an opportunity to set up using Quiver Dance, it can be difficult for offensive teams to defeat due to its added bulk, Special Attack, and Speed, as well as its passive recovery. Its Fairy typing gives it defensive utility via an immunity to Core Enforcer, which would otherwise have the potential to negate Poison Heal, as well as offensive utility by being super effective against some of the more common defensive Pokemon in the tier, Giratina and Zygarde-C. It also gives it useful resistances to both Fighting and Dark, as well as a resistance to U-turn and super effective STAB coverage against Dark and Fighting-types. These can be useful, as the Dark-type resistance allows Xerneas to be a reliable switch into Knock Off to provide utility to the rest of the team, while the Dragon-type immunity and the Fighting-type resistance can allow Xerneas to set up against Choice item users. It also has an immunity to Dragon-type attacks, which can be beneficial. However, Xerneas struggles to break past defensive Steel-type Pokemon, which hinders its offensive capabilities to a noticeable degree due to their prevalence in the metagame. The popularity of Flash Fire Steel-type Pokemon in particular is troublesome for Xerneas, as those Pokemon are able to wall Xerneas with its most common forms of coverage. Furthermore, though Xerneas is able to perform effectively with its Poison Heal set, it is outclassed with practically any other set. It isn't bulky enough to perform well with a purely defensive set, as Mega Audino would be able to, nor does it have the offensive stats to do well with a solely offensive set, as Mega Diancie would be able to.