Gyarados and Flareon best moveset

It depends on your purpose. Gyarados has the strongest Hydro Pump, which is a fine moveset combined with DT or B.

If you wanna counter a psychic pokemon, Bite/Crunch is your choice.

If you wanna counter Dragons, go for DT/O

Basically, don't go for Bite/Outrage or Dragon Tail/Crunch

"Best" moveset comes down to how you want to use your Gyarados.

DT O has the highest DPS in a neutral settting I believe, or very close to DT HP if not.

Good if you want it for sweeping gyms. For raids there are usually better Pokemon or better movesets

For example, Bite Crunch has a niche against a Mewtwo with Focus Blast because Gyarados' flying type resists fighting while Tyranitar is double weak and will probably get one-shot by it.

Flamethrower on Flareon is a great move. Even though does the lowest cycle DPS of the 3 charge moves, all of the moves are close in damage, but it has the advantage of hitting fast, having a low cooldown and being 2 bar.

Sometimes your Pokemon can faint during the charge move animation for a long move like Fire Blast or Overheat, or you faint before you reach 100 energy, where Flamethrower has the advantage, so I would suggest keeping Flamethrower but change Ember to Fire Spin, it's much better.

Have a look at this thread. https://pokemongo.gamepress.gg/q-a/flareon-100-fsft-or-fsoh

B/HP ftw