Heatran

Heatran's excellent typing of Fire / Steel and ability in Flash Fire give it handy resistances to types such as Grass, Fairy, Steel, Dragon, Normal, Psychic, and Ice as well as immunities to Fire and Poison. In addition to the resulting ability to wall these types, it has a wide movepool with a lot of options. For offense, STAB and coverage moves such as Overheat, Flash Cannon, Earth Power, and Rock Tomb let Heatran also hit many Fire-, Poison-, Ice-, Fairy-, and Steel-type Pokemon for super effective damage, such as Charizard and Tapu Lele, and Grassium Z + Solar Beam covers its weaknesses to Water- and Ground-type Pokemon. Along with having fantastic offensive options, Heatran also comes with utility in the form of Metal Sound, letting it beat specially bulky Pokemon such as Blissey and Chansey, and Taunt, allowing it to shut down setup-reliant Pokemon such as Stored Power Necrozma. This is further complemented by Heatran's massive Special Attack stat and solid bulk. However, Heatran is weak to Water-, Fighting-, and Ground-type moves, all common offensive types in the metagame wielded by Pokemon such as Mega Gyarados, Mega Lopunny, and Zygarde-C. On top of that, since it has to use Grassium Z in order to use Solar Beam to cover Water- and Ground-types, it can't use Air Balloon, leaving it vulnerable to Pokemon such as Donphan and Garchomp, which carry ground moves while not fearing an OHKO from Bloom Doom.