Metagross

Mega Metagross is one of the frontrunners of tankiness in 1v1, having the raw bulk and defensive typing to withstand attacks from many of the most viable offensive threats in the metagame, such as Dragonite, Mega Gyarados, Greninja, and Magnezone. Excellent STAB and coverage moves, high Attack, and Tough Claws all combine to make Mega Metagross very competent offensively, as well as making it capable of 2HKOing most opposing Pokemon neutral to its STAB or weak to its coverage moves, like Meloetta, Tapu Fini, and Mega Mawile. Unfortunately, without a viable means of offensively setting up, Mega Metagross finds itself struggling against bulky Pokemon that boost their Defense or reduce the damage it deals, like Double Dance Magearna, defensive Zygarde, and Iapapa Berry Necrozma. Additionally, Mega Metagross cannot afford to be Speed invested to outspeed and OHKO Pokemon that OHKO it like Landorus-T and Garchomp, since it needs to be bulky to withstand hits that would otherwise knock it out, like +1 Mega Gyarados's Crunch, Choice Specs Greninja's Dark Pulse, and Choice Specs Naganadel's Fire Blast.