I caught a Golbat at 22 after Morty so that I could use the Friend Ball and because Zubat is bad in the first 3 or 4 gyms anyway.
As a 30 Crobat it could beat Pryce's Seel and 3HKO's Piloswine who 4HKO's back with Icy Wind but can 2HKO if it hits both Blizzards.
31 Crobat solo's Chuck and loses to Jasmine
32 Crobat sweeps the Rocket Rival fight except Magnemite. It does beat Magnemite 1v1 but is too damaged to sweep after.
37 Crobat beats any of Claire's pokemon 1v1, but takes too much damage to beat more than one.
40 Crobat loses to the Rival's Magneton in Victory Road but can solo the rest of his team
43 Crobat can beat a single of Will's pokemon except for Jynx and Slowbro, taking too much damage to handle more than one of them
43 Crobat sweeps Koga except for his own Crobat, which becomes a very RNG fight but probably favors Koga even if your Crobat is at full health. If I had swift it might have been able to beat Crobat but I gave it to Magnemite
44 Crobat sweeps Bruno if Onix is dealt with. It does beat Onix 1v1 but takes too much damage to sweep everything
44 Crobat beats all of Karen but doesn't sweep unless you switch out to deal with Sand Attack and Curse.
44 Crobat can beat 1 of Gyarados and Charizard but not both while losing to the rest.
As a 30 Crobat it could beat Pryce's Seel and 3HKO's Piloswine who 4HKO's back with Icy Wind but can 2HKO if it hits both Blizzards.
31 Crobat solo's Chuck and loses to Jasmine
32 Crobat sweeps the Rocket Rival fight except Magnemite. It does beat Magnemite 1v1 but is too damaged to sweep after.
37 Crobat beats any of Claire's pokemon 1v1, but takes too much damage to beat more than one.
40 Crobat loses to the Rival's Magneton in Victory Road but can solo the rest of his team
43 Crobat can beat a single of Will's pokemon except for Jynx and Slowbro, taking too much damage to handle more than one of them
43 Crobat sweeps Koga except for his own Crobat, which becomes a very RNG fight but probably favors Koga even if your Crobat is at full health. If I had swift it might have been able to beat Crobat but I gave it to Magnemite
44 Crobat sweeps Bruno if Onix is dealt with. It does beat Onix 1v1 but takes too much damage to sweep everything
44 Crobat beats all of Karen but doesn't sweep unless you switch out to deal with Sand Attack and Curse.
44 Crobat can beat 1 of Gyarados and Charizard but not both while losing to the rest.
Zubat is very good at clearing routes, is good againt Chuck and Clair but bad against Pryce and Jasmine. It beats 3/5 Elite 4 fights with very little support while taking 1 thing down before losing in the other 2. It needs back tracking for the Friend ball and needs Return so while it's a lot better than most of C it still probably belongs there. Without considering the back tracking I'd probably nominate it for B
Took the trade Onix at level 3 and beating every optional trainer before Falkner except Sprout Tower gets you to level 9. At level 9 you have a chance to beat Pidgey with 3 health left and lose to Pidgeotto. If you switch out after Pidgey to reset status you need 1 potion and a berry, if not an extra potion. You can't overlevel Onix to win because it'll stop obeying you
17 Onix sounds over leveled for Bugsy but it gets boosted exp. It doesn't obey so I tried the fight 4 times and only lost once. It feels like it should be fairly consistent. This is the last time Onix disobeys. It also easily beats the Rival, but would have needed a potion if it wasn't Quilava.
20 Onix beats Whitney without even needing to leave Goldenrod and get Dig.
27 Onix is faster and OHKO's all but Gengar. With a mint berry as long as it doesn't hit 2 Hypnosis you win. If it does you need an awakening.
After this Onix beats Primape, Seel and most of the Rival's Pokemon
43 Onix loses to all of Will's pokemon
43 Onix sweeps Koga except for Crobat and Ariados.
44 Onix beats Bruno except Machamp. It takes too much damage to beat more than 2 of them
44 Onix beats Umbreon and Gengar and looks like with another level or 2 beats Murkrow
44 Onix Beats Aerodactyl, even losing to the Thunder Dragonite.
17 Onix sounds over leveled for Bugsy but it gets boosted exp. It doesn't obey so I tried the fight 4 times and only lost once. It feels like it should be fairly consistent. This is the last time Onix disobeys. It also easily beats the Rival, but would have needed a potion if it wasn't Quilava.
20 Onix beats Whitney without even needing to leave Goldenrod and get Dig.
27 Onix is faster and OHKO's all but Gengar. With a mint berry as long as it doesn't hit 2 Hypnosis you win. If it does you need an awakening.
After this Onix beats Primape, Seel and most of the Rival's Pokemon
43 Onix loses to all of Will's pokemon
43 Onix sweeps Koga except for Crobat and Ariados.
44 Onix beats Bruno except Machamp. It takes too much damage to beat more than 2 of them
44 Onix beats Umbreon and Gengar and looks like with another level or 2 beats Murkrow
44 Onix Beats Aerodactyl, even losing to the Thunder Dragonite.
Onix is great early game and awful in the last half. Against route mons you really appreciate the boosted exp because you don't have to use a pokemon with Pidgey's attack stat. Dig's 10 PP is bad when you need so many hits against route mons. You're honestly better off only using Onix to fight Magnemites after Morty and catching a new one before the Elite 4.
Steelix should be D rank, you're either stuck with Onix until Kanto or grinding a metal coat which would be way to RNG dependant, possibly taking a couple hours even with Thief.
Magnemite is easy to level after catching, with many water and flying types in the Lighthouse.
26 Magnemite loses to Morty, beating some of the Ghastly and Haunter but provides great support being able to Thunder Wave Gengar.
30 Magneton beats Pryce's waters while losing to Piloswine
31 Magneton 3HKO's Primape but if it Karate Chops twice you lose. It's faster and 2HKO's Poliwrath while surviving Dynamic Punch so you win ig
you don't hit yourself by holding a berry.
31 Magneton needs Thunder to beat both of Jasmine's Magnemite but beats 1 without.
32 Magneton takes almost 0 damage from the rival, except Quilava who it loses to.
37 Magneton had a chance to barely 1v1 a Dragonair but loses to Kingdra being too slow and getting 2HKO'd
40 Magneton sweeps the rival again excluding Typhlosion.
43 Magneton loses to Exeggutor but beats the rest of Will 1v1, taking too much damage to beat more than 1 or 2
44 Magneton sweeps Koga who just can't touch it.
44 Magneton can beat one of Hitmonlee or Hitmonchan but not both
44 Magneton beats Umbreon, Gengar and Murkrow while usually beating Vileplume, but you can lose to status. Loses to Houndoom
44 Magneton loses to the level 50 Dragonite and Charizard, but can beat any 2 of the others.
26 Magnemite loses to Morty, beating some of the Ghastly and Haunter but provides great support being able to Thunder Wave Gengar.
30 Magneton beats Pryce's waters while losing to Piloswine
31 Magneton 3HKO's Primape but if it Karate Chops twice you lose. It's faster and 2HKO's Poliwrath while surviving Dynamic Punch so you win ig
you don't hit yourself by holding a berry.
31 Magneton needs Thunder to beat both of Jasmine's Magnemite but beats 1 without.
32 Magneton takes almost 0 damage from the rival, except Quilava who it loses to.
37 Magneton had a chance to barely 1v1 a Dragonair but loses to Kingdra being too slow and getting 2HKO'd
40 Magneton sweeps the rival again excluding Typhlosion.
43 Magneton loses to Exeggutor but beats the rest of Will 1v1, taking too much damage to beat more than 1 or 2
44 Magneton sweeps Koga who just can't touch it.
44 Magneton can beat one of Hitmonlee or Hitmonchan but not both
44 Magneton beats Umbreon, Gengar and Murkrow while usually beating Vileplume, but you can lose to status. Loses to Houndoom
44 Magneton loses to the level 50 Dragonite and Charizard, but can beat any 2 of the others.
Routes are ok, taking little damage from a lot of things thanks to steel typing but even low level Grass/Electric/Grounds are very tedious for it. Loses to Morty, Pryce, Jasmine and Clair while being able to beat 1 of Chucks Pokemon, but not sweep. It beats Koga but otherwise only beats the weaker Elite 4 Pokemon. It needs Thunder and Rain dance to be an ok Pokemon but still probably justifies C tier.
28 Seel loses to Chuck but beats Steelix who is 7 levels higher if you have rain dance. Without Rain dance you can do it with a potion.
31 Seel can beat any of the Rivals pokemon but takes too much damage to beat more than one.
36 Dewgong with Blizzard sweeps Clair with an x Accuracy and PRZcure berry. I don't know if 37 Dewgong would KO Dragonair with Ice Beam, but it would need a potion to beat Kingdra.
40 Dewgong beats all the rival's Pokemon 1v1, being able to sweep them all if Magneton is handles by another Pokemon.
43 Dewgong Sweeps all of Will except Slowbro, who it loses 1v1.
44 Dewgong Sweeps Koga
44 Dewgong beats Hitomontop, Hitmonchan and Onix. It needs to hit 2 Blizzard to beat Machamp and be at full health and it loses to Hitmonlee because it outspeeds.
44 Dewgong beats all except for Houndoom, taking to much status/damage to sweep the others though. It's just barely 2HKO's by Houndoom, so
another level or better rolls would have fixed it.
44 Dewgong beats all of Lance 1v1 but takes too much damage to sweep.
31 Seel can beat any of the Rivals pokemon but takes too much damage to beat more than one.
36 Dewgong with Blizzard sweeps Clair with an x Accuracy and PRZcure berry. I don't know if 37 Dewgong would KO Dragonair with Ice Beam, but it would need a potion to beat Kingdra.
40 Dewgong beats all the rival's Pokemon 1v1, being able to sweep them all if Magneton is handles by another Pokemon.
43 Dewgong Sweeps all of Will except Slowbro, who it loses 1v1.
44 Dewgong Sweeps Koga
44 Dewgong beats Hitomontop, Hitmonchan and Onix. It needs to hit 2 Blizzard to beat Machamp and be at full health and it loses to Hitmonlee because it outspeeds.
44 Dewgong beats all except for Houndoom, taking to much status/damage to sweep the others though. It's just barely 2HKO's by Houndoom, so
another level or better rolls would have fixed it.
44 Dewgong beats all of Lance 1v1 but takes too much damage to sweep.
Seel comes late needing Pryce's badge and starts off very bad with base 45 offenses. It evolves late at 34 but you have to wait until 37 if you want Ice Beam for Clair. My 26 Seel lost to a 22 Goldeen and 27 Seel lost to a 24 Raticate, both times being from full health. It would continue to regularly lose against random trainers like this until it evolves. Aurora Beam is not strong enough for Clair, so you're either using Blizzard or having on of the worst Pokemon to baby in the game. It does perform very well against the Elite 4
I think Seel could move down to C but could also stay B. It's questionable if it's matchups in the late game doesn't justify it's availability and how bad it is to raise before it evolves. .