Metagross

Unlike the all-out offensive Mega Evolutions such as Diancie, Charizard Y, and Kangaskhan, Mega Metagross is held back by its poor offensive typing and the low Base Power of its STAB moves. It gives free momentum to the opponent's Steel-types, especially Aegislash, and it can be stopped by common Intimidate users, including Landorus-T. It's also pretty useless after being paralyzed or burned, though unlike Diancie and Charizard Y, it has the bulk to ride out temporary field conditions such as Tailwind and Trick Room. Moreover, with its good defensive typing, high base Speed, and fantastic stats all around, Mega Metagross is still able to serve as an effective speedy attacker, pivot, and utility check to offensive Pokemon such as Keldeo. It's not forced out too easily, except by Aegislash, and it can come in without much difficulty and threaten to deal major damage to offensive Pokemon or potential switch-ins. While the "big three" Mega Evolutions mentioned above may provide immense power, they require a lot of support; Mega Metagross, with its high Speed and numerous resistances, provides both some power and some support and is often a great choice for teams not built around a Mega Evolution.