Mega Steelix: C ----> Unranked
Simply put, there is no reason to ever use this when Mega Aggron is an option.
Aggron has the same Defense, higher Attack, better typing and a better ability. All Steelix has going for it is STAB Earthquake and marginally higher Special Defense.
Just to add to this, before we get the same tired old "it checks volt-turn" arguments, mega-lix is still pretty damn bad at that. The only UU relevant volt-switch mons you block are Rotom-C (who burns you for your trouble), defensive mega-ampharos (offensive ones force you to bet on focus miss living up to its nickname), jolteon (who is garbage in of itself), and galvantula (Who I think doesn't even bother to volt-switch anymore due to the suicide sticky web lead role).
You want to block volt-switching? Run pretty much any other ground type alongside M-Aggron.