Aggron

Mega Aggron is one of the best physical walls in the 1v1 metagame, with amazing defensive stats and serviceable offensive stats that allow it to take hits from and KO a wide range of Pokemon, such as Dragonite, both Mega Charizards, and Aegislash. It can make use of its amazing abilities in Sturdy and Filter to avoid getting KOed while hitting back with powerful moves such as Heavy Slam, Head Smash, and Metal Burst. The combination of Sturdy and Metal Burst allows Mega Aggron to neutralize Choice item users such as Naganadel, Greninja, and Genesect, who would otherwise be too much for Mega Aggron to handle. Mega Aggron also has access to Taunt, which gives it the potential to beat Pokemon that want to play around Metal Burst, like Manaphy, and can shut down certain fast sleep users such as Jumpluff and Vivillon. The need for higher HP and power to beat threats such as Mega Charizard X makes it very vulnerable to specially offensive Pokemon that can play around Sturdy. Metal Burst can also be played around, by either hitting it with a less damaging move in the first turns or using status moves, such as Substitute and Will-O-Wisp. Taunt helps avoid this situation, although it's not very reliable, if they attack when you Taunt. Taunt also isn't enough to prevent losing to common stall threats such as Mega Sableye, Mega Venusaur, and Mega Slowbro, as Mega Aggron has no reliable way of boosting its offenses outside of Curse.

A Rockium Z set can also be used to beat threats to the Mega set, such as both Mega Charizard X and Y, without relying so heavily on Head Smash's inconsistent accuracy. However, Aggron is not nearly as bulky as Mega Aggron and is forced to keep its Rock typing, leaving it susceptible to threats like Mega Metagross, Haxorus, and Mega Lopunny.