Steelix

Steelix's Mega Evolution grants it better Attack, Defense, and Special Defense stats. Most notable is its sky-high base 230 Defense stat, tying with Mega Aggron's. Mega Steelix faces hefty competition from Mega Aggron, which is an arguably better tank because of its ability, Filter; however, Mega Steelix does have more special bulk than Mega Aggron as well as a stronger Earthquake and a far better defensive typing before it Mega Evolves. Mega Steelix's fantastic defensive typing in Steel / Ground gives it a plethora of resistances and two immunities, which let Mega Steelix wall the well-known VoltTurn combination, being immune to Volt Switch and resistant to U-turn. Mega Steelix, however, misses out on the passive recovery that Leftovers would give it. Despite that, Mega Steelix is still a good Pokemon, having access to amazing STAB attacks such as Heavy Slam and Earthquake as well as great utility moves including Stealth Rock and Roar, although it misses out on Thunder Wave, which Mega Aggron gets.