Ttar phantom it's okay! I'm more interested in Poliwag anyway, and Espeon.
- Mid Tier
: Early, Route 16. Evolves at 25, then can evolve to Politoed or Poliwrath (the former is more powerful and King's Rock is easier to obtain, right after you get Surf and Strength). Unfortunately, the former requires trading and the ladder requires getting lucky and obtaining a Water Stone.
: Great either way. Politoed is slower but is more specially bulky and has higher Special Attack.
: Bubble / Water Gun (level 13) are all it gets offensively, but it gets Hypnosis at level 7 which is nice. Later on, it gets the holy triumvirate of HM moves - Surf, Waterfall, Whirlpool, as well as Ice Punch. In Crystal, it gets Ice Beam from the move tutor.
: as a Politoed, has a great base 90 Special Attack and excellent bulk. As Poliwrath, its offense and bulk are more physically oriented. Also gets Body Slam and Belly Drum if you're feeling lucky. Headbutt is great too.
: STAB Water is always great, and the added Fighting typing doesn't give Poliwrath too many relevant weaknesses.
Pryce - Nothing special here, unfortunately. Politoed can't do anything here.
Clair - Politoed wins out here, hitting Clair's 3 Dragonair much harder than Poliwrath could dream of with Ice Punch.
Elite 4 Karen - This sounds crazy, but Politoed can use Perish Song to get rid of Umbreon, but it must use Whirlpool to trap it since the AI WILL switch out Umbreon before the last count.
Blain and Brock - Politoed easily takes these on with little effort, with Surf being super effective against both.
Red - Just as with Karen, you can Perish trap Snorlax.
- High Tier
: Early-mid game, Evolves by happiness and Eevee can't really do anything useful.
: Amazing Special Attack and Speed, but frail physically.
: Only has Swift until you get STAB Psybeam at 36. Gets Psychic at level 47 but it is powerful enough with Psybeam to level up in Kanto.
: Pure Psychic is much better in-game than competitively, and getting STAB on Psybeam and Psychic is Espeon's biggest benefit. No weaknesses except for rare Dark-type attack and nigh non-existent Bug-type attacks.
Won't have Psybeam until after the Elite 4, so it isn't quite useful in Johto.
Various Kanto trainers - will be about the same level or even higher, wrecks with Psybeam, and levels up easily to get Psychic more quickly.
Erika - Psybeam / Psychic destroy her whole team.
Janine - Same story as Erika, perhaps even easier.