Rayquaza

Mega Rayquaza is a dominant force in the current Anything Goes metagame, with its amazing 180 offensive stats coupled with a great Dragon / Flying offensive typing making it one of the best wallbreakers in the tier. Its STAB moves combined with its great coverage make it difficult to find checks and counters that can reliably switch into its attacks. It also has great durability for an offensive Pokemon, as its amazing defensive typing in conjunction with Delta Stream cutting the damage from some super effective moves in half and canceling out opposing Primal weather means it can provide great defensive synergy on offensively oriented teams. Mega Rayquaza has a diverse movepool with priority in Extreme Speed, two setup moves in Dragon Dance and Swords Dance, and amazing coverage letting it hit most of the metagame for at least neutral damage. Its lack of needing to hold a Mega Stone also helps increase the diversity between its sets, meaning it can have more options compared to other Mega Evolutions.

However, although it has a high base Speed, it still does not outspeed key threats in Deoxys-A, Darkrai, and Mewtwo, leaving it reliant on priority to take them out. This is an issue, as Mega Rayquaza is often forced to run Extreme Speed so that it cannot be revenge killed easily, especially before Mega Evolving. Being slower than Arceus also means that it can be taken out by the latter's Extreme Speed. Lastly, Mega Rayquaza takes up your team's Mega slot, preventing you from running other Mega Evolutions.