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Vaporeon

Vaporeon is a somewhat good Water-type with excellent stats, but it can't do much in standard because, like most Water-types, Starmie and Chansey stop it. However, it is pretty good in UU, and is sometimes banned because it's so much better than other UUs stat-wise. Surf and Hydro Pump are good standard attacks, and Blizzard is for Grass-types. Mimic will hopefully get Thunderbolt or something else that could help against Starmie. Body Slam is for paralyzing Starmie or Alakazam, while Rest can work because of Vaporeon's high HP.

Other Options

Acid Armor helps Vaporeon tank. Ice Beam can be used if you are worried about running out of PP with Blizzard or are paranoid of its 90% accuracy. Haze is not very useful in RBY, but it could stop Amnesia Slowbro or Snorlax as a desperation move. Focus Energy should only be used when playing on Stadium. It should be noted that Mimic would be the most likely candidate for replacement with one of these moves, with the obvious exception of Ice Beam.

Opinion

Overall, Vaporeon is a pretty good special attacker; however, other Water-types are generally more useful than it in standard battles.

Counters

Chansey's high HP and Special sponges Vaporeon's special attacks and lets it hit back hard with Thunderbolt. Starmie, Gyarados, and Lapras resist Water-type attacks and also strike back with Thunderbolt. Zapdos possesses the most powerful Electric attacks in the game, and its astronomical Special protects it from Surf and Blizzard; Jolteon is a bit less stable in general, but isn't weak to Ice. Venusaur and Victreebel dislike a massive Blizzard from 110 Special, but a Razor Leaf that always inflicts critical hits deal with Vaporeon in a short time. In UU, Electabuzz, Magneton, and Tangela are good counters. Wigglytuff also works to a certain extent, thanks to its massive HP and ability to use Thunder Wave/Thunderbolt.