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Aegislash
It may not have been in the metagame for as long as some other defining threats, but it sure as hell shaped the metagame while it was legal. It single handedly neutered the viability of dozens of pokemon, and bolstered the viability of pokemon that could answer it - of which there...
Mega Charizard X
Longest standing top-tier viability, potential to threaten almost any team, not much more needs to be said and everything here has been brought up in some other way, shape or form.
To be honest, I'm sure there are very few people here who don't know what KyuB does, and I also find it strange that you gave no mention of it's Life Orb set. On top of that, I find myself repeating this over and over, you haven't explained how the metagame has changed in any way that warrants a...
What KOs would the extra bulk be useful for specifically? If it doesn't avoid any relevant KOs, I don't see any reason not to run max/max - which strictly speaking there isn't necessarily no reason to run either - situational still - but if the bulk doesn't do anything relevant that uninvested...
The main issue with your nomination, regardless of whether I agree with it or not, is that you fail to explain how the metagame has changed for it in a way that warrants a rise. While you've touched upon some of the issues that hold it back, you otherwise haven't really accomplished anything in...