Vaporeon - High / Mid? (C) / Low (GS)

-Availability: Early-mid game, Evolves with a stone so needs some luck there: 'Rare' encounter rate if you would consider it as getting the stone early enough. In GS, you stay Eevee until Kanto, and Eevee is totally useless. That leaves you with an underleveled Pokemon at late-game.
-Stats: Amazing Special Attack, great bulk, a little on the slow side.
-Moves: It gets Surf. What else would you ever need? Aurora Beam at Lv. 36 for complete coverage.
-Type: Two weaknesses, bulky Water-type is the standard for a reason. Great offensively + defensively
-Match-ups:
Useful in caves.
Type Advantage vs. Pryce, Jasmine, also wins vs. Lance and Clair if you have Aurora Beam.
Brock - Surf destroys his whole team
Blaine - Water beats Fire - especially in GSC where there are no coverage options for anything.
Flareon - Mid (C) / Low (GS)

-Availability: Early-mid game, Evolves with a stone so needs some luck there: 'Rare' encounter rate if you would consider it as getting the stone early enough. In GS, you stay Eevee until Kanto, and Eevee is totally useless. That leaves you with an underleveled Pokemon at late-game.
-Stats: Amazing Attack, OK Special Attack. Middling bulk and low Speed holds it back, but it has generally excellent BST.
-Moves: Uses Normal + Fire type coverage. You have access to Headbutt immediately, which hurts from 130 Attack. Sadly, you'll be using Ember until late game.
-Type: Good offensive typing, until you realize it has little STAB options. Defensively, Fire is a shabby typing with common weaknesses.
-Match-ups:
Type Advantage vs. Pryce, Jasmine.
Wins against Erika
Jolteon - Mid / Low (C) / Low (GS)

-Availability: Early-mid game, Evolves with a stone so needs some luck there: 'Rare' encounter rate if you would consider it as getting the stone early enough. In GS, you stay Eevee until Kanto, and Eevee is totally useless. That leaves you with an underleveled Pokemon at late-game.
-Stats: Amazing Speed and Special attack, but it has little bulk. Its unusable Attack is also disappointing, considering most of its level-up and common TM moves are physical.
-Moves: Very, very shallow viable movepool. The only move you will use is Thundershock until late game
-Type: Good offensive typing, until you realize it has little STAB options. Defensively, Electric has little weaknesses, but also little resistances to speak of.
Type Advantage vs. Will, Misty, and that's kind of about it.