Gyarados Best Offensive Moveset is Dragon Tail Outrage, Best Defensive Moveset is Dragon Tail Hydro Pump???

DT/HP is better all the way I think.

Especially on defence, because outrage is easily dodgeable while HP is not, and one hit can KO a Jolteon.

Attacking, only against vaporeon DT/O is preferable but there are many better vaporeon counters. As general attacker, DT/HP is stronger and better for dodging

From my experience, you want X / Hydro Pump on offense, and Dragon Tail / X on defense, where X preferably is Hydro Pump (hard to dodge, and a pain with the dodge glitch) or Crunch (charge spam).

For any mon. all its A-rated movesets are considered to be tied as the best optimal one for it. The order of the movesets has nothing to do with any of them being better or worse than any other one. See "Moveset Grades Explanation" at https://pokemongo.gamepress.gg/moveset-grades-explanation

From my experience, outrage is better on defense. Outrage does a bit less damage than hydro pump per attack, but is a two bar attack, so more damage overall. It fires more sooner and more often. Since hydro pump is a single bar attack, the attacker has a larger range of safe times to use special attacks and a better idea of safe times. For me, the smaller amount of time it takes to fire isn't much of an issue because if this.

I don't have experience with gyarados as a general attacker, but it seems like outrage would be better for similar reasons except in cases where you want to use a water attacker specifically, like against rhydon or a fire defender. Outrage would certainly be better against vaporeon, exeggutor, dragonite, or another gyarados. I would use vaporeon for the cases where you'd want a water attacker.

I have both movesets for Gyarados and I can easily say that DT/HP is better for offense when facing a pokemon that take's neutral damage. DT/OR is extremely good against dragonites however, literally melts them.

I'd vote for DT/Crunch for best defense, then DT/HP then DT/OR.

DT Crunch is in my opinion the most consistent performer as dodging it will not waste the defender gyarados energy because it will come again shortly!

DT HP is hard to dodge but you may luck out and finish the battle before. It is definitely doable with a level 30+ TS/TB jolteon, but this is a horror to prestige against because HP definitely fires before a prestiger can finish the job

DT/OR is a dragonite deterrent but no one uses a dragonite against gyarados anyway...OR is also far too easy to dodge compared to the other two, but I think it does very good damage and comes relatively often (2 bar).

So my verdict is C=HP > (very slightly behind) OR

Hydropump is beast on both att and def

From my experience, a Jolteon can kill a Gyarados before it releases Hydro Pump, assuming the Jolteon has at least 2/3 of the Gyarados's CP.

Dragon Tail + Outrage is the actual best defensive moveset in the versions dodge bug always happens.

Dragon Tail + Crunch is the actual best defensive moveset if your Gyarados's CP isn't above 3000.

Dragon Tail + Hydro Pump is the actual best defensive moveset if your Gyarados's CP is above 3000, and it is in the area where Eevees aren't too common.

The best moveset on attack, does this need explanation? DT/HP is the best generalist moveset for Gyarados.

Personally I find Hydro Pump predictable for dodging, but I have a hard time with Crunch.

Dragon tail and hydropump for sure is the best defence moveset. Most ppl attack Gary with electric pokemon and that combo even they dodge does a lot of damage. I have one at 2800 right now and it's been at the top spot of its gym for almost 2 months. He's a good defender with that moveset keep this in mind though. You need ppl below him for sure