An argument can be made that, with trades, Abra is in its own God Tier. Also, it should be mentioned how useful Teleport is until you get Fly. Abra only needs two Punch moves anyway and you can sub Teleport for Recover or the third Punch later.
Growlithe (C) that high is beyond me. It gets nothing besides Ember and Bite early, has to wait until level 34 for Flame Wheel and level 50 for Flamethrower. Its only level up move as Arcanine is ExtremeSpeed at level 50, therefore negating any utility it would have for getting a Fire Stone before Kanto. It gets Return and Dig but so does half the game. It is Low at best.
Espeon is High, Umbreon is Bottom, Vaporeon, Flareon and Jolteon should be bottom in GS, Sandshrew is Bottom jesus christ that thing is horrible, Flareon (C) and Jolteon (C) for Low. Vaporeon (C) is about right at High.
High
Slowpoke
Avaiability: early
Stats: high
Movepool: good
Advantages: Morty, Jasmine, Clair, Will, Karen somewhat, Lance, Red (Charizard, Venusaur somewhat)
Disadvantages: Price
Gets Surf, Curse, Headbutt, is bulky as hell. Psychic comes very late but Confusion gets the job done until then. While it only evolves on level 37, Slowbro gets Ice Punch and can learn Ice Beam post HoF in Crystal. It is only heavily lacking in Speed. Getting STAB on both Water and Psychic and having the coverage moves it does, Slowpoke is a solid addition to any team.
Vaporeon (C)
Avaiability: early
Stats: above average
Movepool: average
Advantages: Morty, Jasmine, Clair, Will, Karen somewhat, Lance, Red (Charizard, Venusaur somewhat)
Disadvantages: Price
The go-to Water Pokemon if you don't choose Totodile. Good coverage in Surf and Aurora Beam, gets Ice Beam in Crystal, Acid Armor is ocasionally useful. Also learns Waterfall and Whirpool. Absolutely murders Jasmine and Lance, does okay against all other gym leaders and Elite 4 members, though it can't touch Price at all.
High or Upper Mid, not sure:
Drowzee
Avaiability: early
Stats: good
Movepool: wide
Advantages: Team Rocket, Morty, Chuck, Jasmine, Clair, Will, Bruno, Koga, Karen, Lance, Red
Disadvantages: Houndoom
While it is worse than Abra at everything, a bad Abra is still pretty good. Gets STAB on Psychic-type attacks and learns all 3 Elemental Punches which allows it to hit the entire game super-effectively off its decent Special Attack. On one hand it is slow, making it take a lot of unnecessary hits from stuff it OHKOs and doesn't hit as hard as Abra or Espeon and doesn't learn any recovery move. On the other, it is bulkier and gets Hypnosis so it can be your sleep inducer.
Espeon
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Team Rocket, Morty, Chuck, Will, Koga, Bruno
Disadvantages: Jasmine, Karen
Waiting for level 30 to evolve kind of sucks but Psybeam + Bite is decent coverage, and it hits harder and faster than Kadabra while also having access a recovery move in Morning Sun. Missing out on Kadabra's perfect coverage and being kind of a pain to evolve keeps it out of Top.
Low
Jolteon (C)
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Surf area, Price, Will, Lance (non-Dragons)
Disadvantages: Jasmine, Clair
Great stats and avaiability are ruined by a terrible movepool, leaving it stuck with Bite and Thundershock for a long time. It only gets Thunder Wave on level 42, Thunder on level 52 and Thunderbolt after the HoF. It can be levelled easily while surfing but yeah, no.
Flareon (C)
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Jasmine, Price
Disadvantages: Clair, Lance, wild Geodude
Notoriously bad for having the wrong base stats, stuck with Ember until level 42 (I mean, come on!), slow, terrible typing... I could go on and on. It is only not in bottom because Return from base 130 Attack still hurts like hell and Bite gives it SOME coverage.
Zubat
Avaiabilityle: early
Stats: above average
Movepool: poor
Advantages: Rockets, Chuck, Koga, Bruno
Disadvantages: everything else
Painful to level up (yay Leech Life), gets no STAB whatsoever outside of Wing Attack and Fly, and takes forever to get to its final stage, which isn't even that good anyway. Crobat is fast and the Rockets can barely touch it, murders Chuck and that is it. Also, Zubat is outclassed by the Golbats in Mt. Mortar, which come with Screech (in Crystal at least).
Marill
Avaiability: mid way
Stats: poor
Movepool: poor
Advantages: none
Disadvantages: anything with a strong STAB, super-effective move or a semblance of Special Defense
Boy, can this thing tank. Unfortunately it has terrible attacking stats and Speed so its great bulk only delays the inevitable. On the bright side, it does learn Strenght, Surf, Waterfall and Whirpool, so it is a good HM Slave.
Bottom
Umbreon
Avaiability: mid way
Stats: above average
Movepool: shallow
Advantages: pretty much nothing
Disadvantages: pretty much everything but particularly Chuck and Bruno
Can take hits all day but struggles to deal damage outside of Toxic and Confuse Ray and is very slow. Flareon is better (so is Ledian).
Sandshrew
Avaiability: early
Stats: poor
Movepool: poor
Advantages: nothing
Disadvantages: everything
Gets only 3 STAB moves in the entire game: Mud-Slap, Dig and Earthquake. Its only other move of note is Slash. Terrible stats all around coupled to that laughable movepool make it one of the worst in game pokemon of all generations.
Growlithe (C) that high is beyond me. It gets nothing besides Ember and Bite early, has to wait until level 34 for Flame Wheel and level 50 for Flamethrower. Its only level up move as Arcanine is ExtremeSpeed at level 50, therefore negating any utility it would have for getting a Fire Stone before Kanto. It gets Return and Dig but so does half the game. It is Low at best.
Espeon is High, Umbreon is Bottom, Vaporeon, Flareon and Jolteon should be bottom in GS, Sandshrew is Bottom jesus christ that thing is horrible, Flareon (C) and Jolteon (C) for Low. Vaporeon (C) is about right at High.
High
Slowpoke
Avaiability: early
Stats: high
Movepool: good
Advantages: Morty, Jasmine, Clair, Will, Karen somewhat, Lance, Red (Charizard, Venusaur somewhat)
Disadvantages: Price
Gets Surf, Curse, Headbutt, is bulky as hell. Psychic comes very late but Confusion gets the job done until then. While it only evolves on level 37, Slowbro gets Ice Punch and can learn Ice Beam post HoF in Crystal. It is only heavily lacking in Speed. Getting STAB on both Water and Psychic and having the coverage moves it does, Slowpoke is a solid addition to any team.
Vaporeon (C)
Avaiability: early
Stats: above average
Movepool: average
Advantages: Morty, Jasmine, Clair, Will, Karen somewhat, Lance, Red (Charizard, Venusaur somewhat)
Disadvantages: Price
The go-to Water Pokemon if you don't choose Totodile. Good coverage in Surf and Aurora Beam, gets Ice Beam in Crystal, Acid Armor is ocasionally useful. Also learns Waterfall and Whirpool. Absolutely murders Jasmine and Lance, does okay against all other gym leaders and Elite 4 members, though it can't touch Price at all.
High or Upper Mid, not sure:
Drowzee
Avaiability: early
Stats: good
Movepool: wide
Advantages: Team Rocket, Morty, Chuck, Jasmine, Clair, Will, Bruno, Koga, Karen, Lance, Red
Disadvantages: Houndoom
While it is worse than Abra at everything, a bad Abra is still pretty good. Gets STAB on Psychic-type attacks and learns all 3 Elemental Punches which allows it to hit the entire game super-effectively off its decent Special Attack. On one hand it is slow, making it take a lot of unnecessary hits from stuff it OHKOs and doesn't hit as hard as Abra or Espeon and doesn't learn any recovery move. On the other, it is bulkier and gets Hypnosis so it can be your sleep inducer.
Espeon
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Team Rocket, Morty, Chuck, Will, Koga, Bruno
Disadvantages: Jasmine, Karen
Waiting for level 30 to evolve kind of sucks but Psybeam + Bite is decent coverage, and it hits harder and faster than Kadabra while also having access a recovery move in Morning Sun. Missing out on Kadabra's perfect coverage and being kind of a pain to evolve keeps it out of Top.
Low
Jolteon (C)
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Surf area, Price, Will, Lance (non-Dragons)
Disadvantages: Jasmine, Clair
Great stats and avaiability are ruined by a terrible movepool, leaving it stuck with Bite and Thundershock for a long time. It only gets Thunder Wave on level 42, Thunder on level 52 and Thunderbolt after the HoF. It can be levelled easily while surfing but yeah, no.
Flareon (C)
Avaiability: early
Stats: high
Movepool: shallow
Advantages: Jasmine, Price
Disadvantages: Clair, Lance, wild Geodude
Notoriously bad for having the wrong base stats, stuck with Ember until level 42 (I mean, come on!), slow, terrible typing... I could go on and on. It is only not in bottom because Return from base 130 Attack still hurts like hell and Bite gives it SOME coverage.
Zubat
Avaiabilityle: early
Stats: above average
Movepool: poor
Advantages: Rockets, Chuck, Koga, Bruno
Disadvantages: everything else
Painful to level up (yay Leech Life), gets no STAB whatsoever outside of Wing Attack and Fly, and takes forever to get to its final stage, which isn't even that good anyway. Crobat is fast and the Rockets can barely touch it, murders Chuck and that is it. Also, Zubat is outclassed by the Golbats in Mt. Mortar, which come with Screech (in Crystal at least).
Marill
Avaiability: mid way
Stats: poor
Movepool: poor
Advantages: none
Disadvantages: anything with a strong STAB, super-effective move or a semblance of Special Defense
Boy, can this thing tank. Unfortunately it has terrible attacking stats and Speed so its great bulk only delays the inevitable. On the bright side, it does learn Strenght, Surf, Waterfall and Whirpool, so it is a good HM Slave.
Bottom
Umbreon
Avaiability: mid way
Stats: above average
Movepool: shallow
Advantages: pretty much nothing
Disadvantages: pretty much everything but particularly Chuck and Bruno
Can take hits all day but struggles to deal damage outside of Toxic and Confuse Ray and is very slow. Flareon is better (so is Ledian).
Sandshrew
Avaiability: early
Stats: poor
Movepool: poor
Advantages: nothing
Disadvantages: everything
Gets only 3 STAB moves in the entire game: Mud-Slap, Dig and Earthquake. Its only other move of note is Slash. Terrible stats all around coupled to that laughable movepool make it one of the worst in game pokemon of all generations.