Gyarados

Water

Access to a great setup move in Dragon Dance, a plethora of utility moves including Taunt and Substitute, and a fearsome Attack stat all help to set Gyarados apart from other physically offensive Pokemon on Water-type teams. With Intimidate to help it take physical hits even better, Flying-type coverage to beat Grass-types, notably Mega Venusaur, and a Ground-type immunity that Water teams have a lack of, Gyarados proves itself to be a valuable member to any Water-type team. That being said, its weakness to Stealth Rock and a 4x weakness to Electric hold it back. It suffers from four-moveslot syndrome, as it has access to excellent coverage moves, including Earthquake and Ice Fang, yet to use those, it has to miss out on either Substitute, which makes setting up much easier, or one of its STAB moves. It is unable to make use of its various good defensive traits, such as its helpful Ground-type immunity, its ability Intimidate, and its decent bulk, because of a lack of reliable recovery.

Upon Mega Evolving, Mega Gyarados gains Mold Breaker, letting it break through annoying abilities such as Sturdy and Unaware. It gains an increase in defensive stats that helps in creating more opportunities to set up. It boasts a very high Attack stat coupled with a STAB combination that can create pressure on any team lacking a Pokemon that resists both. STAB Crunch in particular threatens Psychic-type teams greatly and limits possible switch-ins. Despite that, Mega Gyarados has a mediocre Speed stat when unboosted and gets worn down rather quickly because of its weakness to Stealth Rock while non-Mega. It also loses its immunity to Ground and becomes weak to Bug-, Fairy-, and Fighting-type moves.