LO Gyarados. There are no electric priority moves. Your basic set, Dragon Dance / Aqua Tail / Stone Edge / Ice Fang has very few counters. BulkyGyara can't tank and can't sweep, so its a no go.
Gyarados' counters:
- Celebi - 2HKOed by LO Ice Fang after a DD, and Grass Knot fails to KO Gyarados before it wipes out Celebi. Reflect variants also are at risk if they are 77% health or below on the switchin (SR + 11% prior damage), or if Gyarados gets lucky and rolls more than minimum damage the two turns
- Tangrowth - LO Gyarados does min 44% to Tangrowth meaning it would wipe Tangrowth out before it can KO it. Unless Tangrowth is running Morning Sun it doesn't have a chance.
- Cresselia - LO Aqua Tail does 48% min to 252 HP/108 Def Bold so it is a 2-3HKO. Cress can't KO back with Charge Beam in return. Unless it has Moonlight / Reflect, it doesn't stand a chance.
- Uxie - No reliable recovery besides RestTalk meaning that it is prone to being worn down, DD Aqua Tail does min 51% and max 61%.
- Mesprit - Doesn't carry recovery at all in most cases meaning that it can be worn down, LO DD Aqua Tail does min 58%
- Starmie - Gyarados outspeeds and does 86% min with Stone Edge/EQ, OHKOs with Bite on the standard spread. Max defense variants take 67% min from Bite and 55% min from Stone Edge / EQ
- Slowbro - 2HKOed by LO DD Bite after SR damage, can't OHKO back, otherwise it can counter Gyarados
- Milotic - 2HKOed by LO DD Stone Edge / EQ while failing to OHKO in return
- Vaporeon - Counter
- Suicune - Counter
- Porygon2 - 100% counter
Not to mention with the speed boost Gyarados would be able to outspeed a lot of common pokemon and Gyarados can setup on a whole host of pokemon.
Garchomp is prepared for on every team, whereas Gyarados can't really be prepared for unless said team has a bulky counter. In the case of all-out offense teams they get brutally decimated by LO DD Gyarados and mostly the only thing stopping it is Deoxys-S.