From the very beginning I thought Gyarados should have been Water/Dragon, but I think Nintendo didn't want to have a weakness-less (At the time) Pokemon. It belongs to that selection of Pokemon like Rhyperior that have a great special movepool and terrible SpA. With a limited physical movepool it has only a few sets, based around similar ideas:
Gyarados @ Lum Berry
ADAMANT
+Intimidate+
252 Atk/252 Spe/4 Hp
Dragon Dance
Aqua Tail
Stone Edge
Earthquake
As submitted by Tenodera:
->Standard<-
Gyarados @ Wacan Berry / Life Orb / Leftovers / Eject Button
ADAMANT
+Intimidate+
252 HP / 252 Atk / 4 SpD
-Waterfall
-Earthquake / Stone Edge / Avalanche
-Thunder Wave / Taunt
-Protect / Taunt
Gyarados' base 81 Spe stat suggests that it will try to troll base 80s and KO things with it, but Gyarados finds itself in a surprising supporting role in GBU Doubles. If your partner is unable to outspeed a foe that it could otherwise KO, have it Protect while Gyarados T-Waves, then beat it up Turn 2. Waterfall is obvious, and its flinch rate can combine with paralysis to stop the foe from moving about 40% of the time, but the auxiliary non-STAB move is up to the user. Earthquake is potent, but very ally-dependent; Stone Edge is powerful, and inaccurate; Avalanche (Or Ice Fang) hits 4x weak Dragon-types and leaves Gyarados resisted only by Water-types, but also suffers from (normally) low base attack power.
The item is another matter for debate. Wacan Berry almost guarantees Gyarados will survive an Electric-type attack short of the use of an Electric Gem, but if the Electric move was weak and was only going to do about 85% or so to Gyarados sans Wacan, it will be a 2HKO one way or the other. At the far end of ordinary viability, the Eject Button gives Gyarados the opportunity to renew its Intimidate, but can backfire if he's hit first turn before he fires off a Thunder Wave or Taunt.
->Defensive<-
Gyarados (F) @ Wacan Berry
Trait: Intimidate
EVs: 252 HP / 52 Atk / 140 Def / 20 SDef / 44 Spd
Adamant Nature (+Atk, -SAtk)
- Thunder Wave
- Waterfall
- Taunt
- Protect
As submitted by Human:
Gyarados -> Single Battles
Gyarados @ Lum Berry
ADAMANT
+Intimidate+
252 Atk/252 Spe/4 Hp
Dragon Dance
Aqua Tail
Stone Edge
Earthquake
As submitted by Tenodera:
I've taken to using Lum Berry on offensive DD Gyarados. It allows you to set up multiple DDs on many bulky Waters, since most of them stay in to try and get a burn with Scald. Really only useful in Singles though.
Gyarados -> Double Battles
->Standard<-
Gyarados @ Wacan Berry / Life Orb / Leftovers / Eject Button
ADAMANT
+Intimidate+
252 HP / 252 Atk / 4 SpD
-Waterfall
-Earthquake / Stone Edge / Avalanche
-Thunder Wave / Taunt
-Protect / Taunt
Gyarados' base 81 Spe stat suggests that it will try to troll base 80s and KO things with it, but Gyarados finds itself in a surprising supporting role in GBU Doubles. If your partner is unable to outspeed a foe that it could otherwise KO, have it Protect while Gyarados T-Waves, then beat it up Turn 2. Waterfall is obvious, and its flinch rate can combine with paralysis to stop the foe from moving about 40% of the time, but the auxiliary non-STAB move is up to the user. Earthquake is potent, but very ally-dependent; Stone Edge is powerful, and inaccurate; Avalanche (Or Ice Fang) hits 4x weak Dragon-types and leaves Gyarados resisted only by Water-types, but also suffers from (normally) low base attack power.
The item is another matter for debate. Wacan Berry almost guarantees Gyarados will survive an Electric-type attack short of the use of an Electric Gem, but if the Electric move was weak and was only going to do about 85% or so to Gyarados sans Wacan, it will be a 2HKO one way or the other. At the far end of ordinary viability, the Eject Button gives Gyarados the opportunity to renew its Intimidate, but can backfire if he's hit first turn before he fires off a Thunder Wave or Taunt.
->Defensive<-
Gyarados (F) @ Wacan Berry
Trait: Intimidate
EVs: 252 HP / 52 Atk / 140 Def / 20 SDef / 44 Spd
Adamant Nature (+Atk, -SAtk)
- Thunder Wave
- Waterfall
- Taunt
- Protect
As submitted by Human:
44 Speed to outspeed Cresselia. 20 SDef to always survive Zapdos/Thundurus thunderbolt assuming Modest. Max HP obvious bulk. 140 for physical defense and 4HKOd by Terrakion Rock Slide 50% of the time, generally useful to have physical bulk when attack EVs aren't too important. 52 attack EVs just obligatory offense. This is for Doubles.