SharpBlue

Competitive Gyarados

Item: Leftovers  
Ability: Intimidate  
Level: 50  
EVs: 84 HP / 244 Atk / 4 Def / 4 SpD / 172 Spe  
Jolly Nature  
- Waterfall  
- Bounce 
- Substitute  
- Dragon Dance

This is Gyarados's bread-and-butter set this generation, being able to set up on a multitude of Pokemon. Dragon Dance is just an amazing move for Gyarados, boosting its Attack to unbelievably high levels and patching up its underwhelming Speed. Gyarados's many useful resistances give it plenty of opportunities to set up, as does Intimidate, which allows it to take various neutral attacks such as Ice Shard, Icicle Crash, and Crunch. On top of this, Substitute enables Gyarados to avoid status, revenge killers, and Volt Switches. In a metagame full of those three things, Substitute makes it much less of a burden to set up. All walls that don't carry phazing moves are going to be in trouble against Gyarados.

As for the attacking moves, Gyarados's STABs generally provide enough coverage. Waterfall, especially with a rain boost, can put heavy dents in any Pokemon unlucky enough to be on the receiving end of it. Bounce has nice coverage with Waterfall, most notably hitting bulky Water-types, and ensures Gyarados's Substitute isn't broken while it attacks, only faltering against Thunder in rain while Gyarados is in the air.

The EVs allow Gyarados to outrun Jolteon at +1 and prevent a burned Ferrothorn from breaking a Substitute with Power Whip. The HP number achieved increases the healing received from Leftovers, and the remaining EVs are pumped into Attack for more power. Waterfall is obviously irreplaceable on this set, but Bounce isn't. Stone Edge can be used to hit opposing Gyarados, and Dragonite harder, but Bounce is generally more useful.

The biggest threats to Gyarados is Ferrothorn and defensive Politoed / Vaporeon. Defensive Politoed / Vaporeon has Protect to foil Bounce and can tank a few Waterfalls (Vaporeons Water Absorb) due to its resistance, while using Perish Song (Politoed) to force Gyarados out. Finally, Ferrothorn can use Leech Seed the turn that Gyarados hits it with Bounce (as long as Gyarados isn't behind a Substitute) and then slowly stall it out, with Power Whip and Iron Barbs further helping drain Gyarados's life, or just slowly kill it with Power Whip and even shut down Bounce if Ferrothorn has Protect.