Latios

Latios is the premier check to Groudon and Kyogre, the two most threatening and important Pokemon in the tier, making it nigh impossible for teams to pass; unless they run Latias instead. Latios is also a top offensive threat itself; Soul Dew effectively gives it a Calm Mind boost, and it is faster than the majority of Pokemon in the tier. Its immunity to Ground and Spikes and resistances to Fighting and Electric are important defensive qualities, increasing Latios's versatility and capacity to come into play. Latios is especially potent with rain support, as its powerful Thunder either heavily damages many of its checks or inconveniences them with its high paralysis rate. Latios is, however, a victim of its own reputation and influence; its prevalence as a key threat forces many teams to explicitly prepare for Latios, denying its standard sets from making substantial progress against them, especially through the use of Blissey and sun. Also, Latios's poor physical bulk means the likes of Deoxys-A, Salac Berry Groudon, and Extreme Speed Rayquaza can revenge kill or sweep through it quite effectively. Latios is also vulnerable to losing Speed ties and Calm Mind wars in some late-game scenarios.