Gyarados

Armed with Dragon Dance, a reliable physical Water-type STAB attack, and two incredible abilities, Gyarados establishes its niche as a dangerous sweeper. Utilizing its solid bulk and rare typing, Gyarados has many switch-in and setup opportunities, pivoting into and threatening top Pokemon like Excadrill, Heatran, Gliscor, Volcarona, Scizor, and Keldeo. Many of the Pokemon it checks are common on hyper offense teams, which grants it a particularly strong matchup against them. The ever-present rain set by Politoed is also a boon to Gyarados, as it clears the sand that cuts into its bulk, and physical checks like Skarmory and Reuniclus can be much more easily muscled through with a boosted Waterfall when rain is present.

However, weakness to Stealth Rock and vulnerability to sand severely cuts into Gyarados's defensive reliability, necessitating entry hazard removal support. This is exacerbated by it relying on its bulk and typing to set up directly on Pokemon like Excadrill. Gyarados is also not very immediately threatening, as its middling Speed and mediocre STAB option of Waterfall often require it to use Dragon Dance before it can meaningfully damage teams. This Speed issue persists even after setting up one Dragon Dance, as common Choice Scarf users like Latios and Garchomp often put Gyarados in an awkward situation after it has claimed one KO. Finally and most importantly, Gyarados has a large number of checks, such as Skarmory, Jellicent, Ferrothorn, Rotom-W, Encore Politoed, and Dragonite, which are all quite common and often necessitate different moves to beat.