Gyarados

Mega Gyarados has carved a niche for itself in this metagame thanks to Mold Breaker, which is excellent against users of Multiscale, Sturdy, and Magic Bounce and thus makes it an answer to defensive teams. It's also decent versus hyper offensive teams because it works as a secondary check to Primal Groudon and Arceus-Ghost, having access to Intimidate and a Ground immunity before Mega Evolving. Finally, its decent 95 / 109 / 130 bulk and sheer physical power given by a base 155 Attack stat greatly help it fulfill its duties. However, Mega Gyarados is vulnerable to Xerneas and Mega Salamence, both very popular Pokemon. It also removes the possibility of running Mega Salamence, which is much better against hyper and bulky offensive teams. Lastly, it does not contribute to the defensive synergy of the team outside of checking offensive Arceus-Ghost and some variants of Primal Groudon; it even has to be cautious of the latter's Lava Plume.