Steelix

Mega Steelix is a great tank that takes advantage of its great bulk, good typing, and high Attack to check prominent threats like Latias, Heal Bell Togekiss, and Mega Altaria. Coupled with access to Stealth Rock, these traits differentiate Mega Steelix from other Stealth Rock setters such as Nihilego and Hippowdon. Mega Steelix's Ground typing allows it to block Volt Switch, which gives it leverage over other Steel-types like Cobalion and Stakataka. That said, Mega Steelix still suffers from heavy competition with Mega Aggron, which has access to Filter and has more resistances. Furthermore, despite having eight resistances, Mega Steelix suffers from a weakness to common types including Fighting, Water, and Ground, which leaves it weak to other prominent threats, including Terrakion, Primarina, and Hippowdon. Mega Steelix's lack of reliable recovery also lessens its reliability, as it is forced to be paired with Wish support if it desires to reach its full potential as a wall. While Mega Steelix's low Speed increases Gyro Ball's damage output, it also leaves Mega Steelix outsped by the vast majority of the metagame, which means it'll have to take a hit before moving in most cases. Finally, despite its Ground typing, Mega Steelix struggles to reliably check common Electric-types like Mega Manectric and Rotom-H because they can hit it super effectively. Hence, other Ground-types like Hippowdon and Swampert outperform Mega Steelix in this role.