Aggron

Aggron sees considerable improvements in multiple key areas upon Mega Evolution, making it a solid pick in the UU metagame. Most notably, its Defense stat boosts to a gargantuan base 230, pairing nicely with its solid defensive typing in Steel and making it a strong answer to Pokemon like Mega Aerodactyl and Dragon Dance Mega Altaria. Filter makes for a fantastic ability, helping it against super effective attackers like Krookodile and Cobalion. Even uninvested, Mega Aggron's base 140 Attack stat gives it respectable offensive presence for a more bulk-oriented Pokemon, especially when combined with its naturally powerful Heavy Slam and nice coverage options in Earthquake and Fire Punch. However, its HP and Special Defense stats are rather mediocre, limiting its overall effectiveness as a tank. Additionally, Mega Aggron has no passive or reliable recovery, making it easily worn down over the course of a match. Mega Steelix also poses stiff competition as a Steel-type Stealth Rock user due to its similar stats and immunity to Electric, though Mega Aggron does possess a better ability in Filter and a generally more useful defensive typing. Lastly, Mega Aggron faces poor one-on-one matchups against some Defog users such as Gligar, Moltres, Rotom-H, and Mew, which can pose formidable obstacles to keeping up Stealth Rock.