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• Moves

Gyarados

Gyarados has one the highest overall stats of all Pokémon, but people brush it off because of its 4x weakness to Electric-type attacks. Thunderbolt hurts Starmie on the switch and Blizzard hurts Exeggutor a little. Gyarados' Body Slam and Hyper Beam are quite powerful too. Surf can OHKO Rhydon and Golem, but Blizzard is generally better because it also threatens Ground-types and has better type coverage.

Other Options

Ice Beam works if you prefer accuracy and Thunder or Hydro Pump if you don't. Flamethrower and Fire Blast are up there, but Gyarados already hits just about everything quite hard—the only things you'll be missing out on hurting super effectively are Bug-types, who are not that much of a threat. Substitute and Mimic should always be mentioned even though they're usually too situational. Leer can be handy sometimes for forcing switches or putting something like Chansey or a paralyzed Tauros in Hyper Beam range, but don't expect too much of it.

Opinion

If Gyarados didn't learn Thunderbolt it'd probably be somewhere along with the likes of Flareon and other great Pokémon that are ignored because of their poor movepools, but it does, and that makes it a solid Pokémon. Basically, Gyarados has a neat enough Attack that it can do major damage, and a diverse movepool that boosts the value of a very nice special stat. Just watch out for OTHER Pokémon with Thunderbolt; the weakness to such a common move certainly hurts it in the standard environment, but it's still a usable Pokémon.

Counters

Starmie, Lapras, and Gengar have Thunderbolt and will use it from their good Special stats, which spells doom for Gyarados, who is four times weak to the move. Gengar, in addition, isn't affected by Gyarados' Normal attacks. Also, since Gyarados doesn't get STAB on its most useful attacks, a high HP Pokemon like Clefable or Snorlax would be able to handle it more often than not.