Fair enough. Will do sample write ups for Spearow and Gyarados with that in Mind.
Gyarados to S
Availability: Magikarp can be found as early you get the Old Rod but doing this means you have to baby Magikarp around until level 20. The Red Gyarados comes in at level 30 mid way through the game at the Lake of Rage. This is objectively the stronger version of the Pokemon, so keep that in mind when considering it for your team.
Stats: Magikarp is awful. Gyarados however has amazing stats across the board. It's lowest stat is its special attack, which is still decent at 60 for the early game. Gyarados has outstanding bulk and power and an amazing ability in Intimidate.
Typing: Magikarp is pure Water, while Gyarados is Water and Flying. That Flying Stab will not be used in game, but does help defensively.
Movepool: Magikarp is useless with Splash until level 15 where it gets Tackle. Once it evolves at Level 20 Gyarados gets Bite. Dragon Rage at 23 will basically 2HKO for most of the mid game. Gyarados learns Leer at 26 and Twister at Level 29. The Red Gyarados comes with all of the past four moves. Ice Fang at 32 is amazing, and will help you deal with Dragons later in the game while being good coverage. Aqua Tail at 35 is a good STAB option to use Physically. Rain Dance and Hydro Pump come at 38 and 41 respectively. The real kicker is Dragon Dance at 44, which boosts Gyarados's Attack and Speed and lets it singlehandedly sweep the Elite 4. TM and HM Wise Surf can be used on Gyarados but comes from its less powerful Special Attack. Waterfall however will be your main Stab once it is obtained.
Major battles:
Bugsy: The first three gym leaders assume that you did choose to raise Magikarp up. Outside of switching in to gain exp, Magikarp is close to useless in this gym. If your Magikarp has Tackle it can take on Metapod and maybe Kakuna if it does not get poisoned. You will never beat Scyther.
Whitney: Magikarp is useless here. If you do have Gyarados though it does strongly here. If your Gyarados is Female, the only concerns you should have against Miltank is Rollout and flinches. You are very bulky and Intimidate does weaken Miltank a lot.
Morty: Bite Gyarados can take on everything here. It does get outsped due to how fast Gengar and Haunter are, but it has the power to quickly take them down and eat a shadow ball if needed.
Chuck: From this point onward, you have access to the Red Gyarados. It will preform the same except it is probably higher level. Gyarados resists both Primeape and Poliwrath's fighting STAB. Watch out for Primeape's Rock Slide and Poliwrath's Water Absorb.
Jasmine: Do not send Gyarados out against the Magnemites. Against Steelix you can kill it faster than it kills you with Surf or Aqua Tail.
Pryce: While Gyarados does not resist Ice Beam, it is bulky enough to tank any hits needed and attempt to muscle through with Bite and Dragon Rage if it still has it.
Clair: Ice Fang and your massive stats let you tank any hits that would come your way. It is advised not to lead with it against her own Gyarados due to its own Intimidate. The only real cause for alarm is Kingdra, who quadruple resists your Water attacks and can spam Smokescreen. You still are strong enough physically to push onward with Ice Fang.
Will: Dragon Dance up on his lead and click Bite.
Koga: Dragon Dance up on his lead and click Waterfall
Bruno: Dragon Dance up and click Waterfall again
Karen: Dragon Dance up and click Waterfall and Ice Fang
Lance: I think there might be a pattern here. DD up and click Ice Fang and Waterfall.
Additional Notes: If you are not using the Red Gyarados, it makes an amazing HM Slave. It's access to Surf, Strength, Rock Smash, Whirlpool, and Waterfall make it a strong HM Slave to help get around Johto.
Spearow to S
Availability: Spearow can be found as early as Route 33, but it is advised you speak to the guard outside of Goldenrod City on Route 35. He gives you his Spearow, Kenya, hoping that you will get his friend on Route 31. This man will accept the letter on any Pokemon and not keep them if they are not Kenya, so feel free to add Kenya to your team! Kenya comes with a trade bonus of exp and at level 20, making it quite strong and close to evolving!
Stats: Solid offensive stats. In game that attack and speed is decent enough. Bulk wise it leaves some room to be desired, but is still passable.
Typing: Normal Flying is good in game, since there are so few Rock and Steel types to resist your main stab.
Movepool: Kenya will come with Leer, Fury Attack, Aerial Ace, and Pursuit. Leveling it up one level causes it to Learn Mirror Move and evolve. From there, the only stand out moves are Assurance at 35, Roost at 41 (which you have the TM for), and Drill Peck at 47. TM wise, Spearow is an amazing choice for Return, since it gives it an amazing normal STAB. Fly can be used to give Spearow another STAB option besides Aerial Ace. U-Turn is decent coverage if needed. Steel Wing is also decent but it comes too late to really be useful.
Major battles:
Bugsy: This is the only fight without Kenya. Spearow needs to grind a bit since it only comes at level 6, but it does decently against Bugsy. Scyther hits hard but it is still manageable.
Whitney: Kenya is always a female, so there is no Attract shenanigans. Clefairy is easy provided it does not get lucky with Metronome. Miltank though is a different beast. Even when evolved Kenya will have issues here, since Miltank is so bulky and has Rollout.
Morty: Your normal type will make it so Morty does not spam Shadow Ball, letting you have a strong matchup. Just make sure to switch when he Curses Kenya.
Chuck: Aerial Ace does good work here. However, both mons have their own tricks up their sleeve to deal with you. Primeape's Rock Slide is dangerous, while Poliwrath's Hypnosis is a death sentence. Keep in mind that you do not resist their Fighting STAB as well.
Jasmine: Kenya ain't doing jack here.
Pryce: While you deal good damage with Return, Pryce can easily 2hko you at worst with Dewgong and Pilowsine.
Clair: Do not lead against Gyarados. That Intimidate will make Kenya so much weaker. Kenya can easily take on the Dragonairs, although being parylyzed is annoying. Kingdra is interesting, as Aerial Ace does help you avoid her inevitable Smokescrean, but her Hydro Pump and Dragon Pulse will be devastating for you.
Will: Your weak Assurance is not going to sweep him. You can easily beat everything but Slowbro by just clicking Return and your STAB against Exeggutor.
Koga: Stab spam again, just watch out for Forretress. Crobat also has the dubious honor of being one of the few pokemon that reliably outspeeds you.
Bruno: Against everything except Steelix you should do well provided you are not super underleveled. Just click your stab and clean house.
Karen: Stab spam works ok here. Gengar and Vileplume are your best matchups.
Lance: You will not 1v1 any of the Dragonites. You only can beat Charizard.
Additional Notes: If you choose to use non Kenya Spearow, be advised it is weaker and does not have that sweet trade bonus exp. Kenya is an outstanding Route clearer as well.
Ryota Mitarai that look good?
BTW here is what imo the S Tier should look like
Totodile
Gyarados
Cyndaquil
Alakazam
Heracross
Spearow