Please also do not reserve any Pokemon ranked D or below as they will likely not receive a writeup.
It feels kinda weird to leave out D Rank when the B2W2 in-game tierlist included their D Rank. What's more, B2W2 included all of their ranks (S to D, no E) and I'm pretty sure they have even more Pokemon than this tierlist, so I don't really know why this exclusion is needed, outside of it just being a "preliminary adjustment" sort of thing.
Edit:
Merritt I already have write-ups ready for Gulpin, Grimer, and Koffing, but I guess I'll just bring them up once all the S to C write-ups are done.
Name: Pikachu
Availability: Safari Zone, Area 1 & 2 Grass, 15%, Levels 25 or 27
Stats: High Speed but lackluster stats everywhere else. Offenses improve upon evolution or using the Light Ball item.
Typing: Electric typing has great use against Winona, the upcoming water routes, and Water-type bosses such as Juan, Wallace, and Glacia.
Movepool: Level 25 Pikachus come with Thunder Wave and learn Thunderbolt in 1 level, whereas Level 27 Pikachus already come with Thunderbolt, but needs to be reteached Thunder Wave, so take your pick on which is more valuable for you. As an Electric-type, Pikachu's movepool is pretty sparse, but it does get Brick Break and Light Screen to improve its coverage and utility, respectively. Because Pikachu doesn't need anything from its level-up movepool that can't be taught by TM, it is advised to evolve it immediately.
Major Battles: A quick backtrack lets Raichu grind for experience against Winona's Gym and take out everything except for Winona's Altaria. Raichu can fry most of the Water-types of Juan, Wallace, and Glacia and use Light Screen to help buffer hits better. Thunder Wave can help to neuter Drake's Salamence, while Brick Break has some mild use against Glacia's Light Screen Glalie as well as Sidney in general.
Additional Comments: While Pikachu does not need the Thunderbolt TM, it is still necessary to travel through New Mauville to acquire the Thunderstone, unless you're lucky enough to catch a Pikachu holding a Light Ball. One could also manipulate Pikachu's nature with the Pokeblock feeder, with dry berries increasing the likelihood of +SpA natures and sweet berries making +Spe natured Pikachus more common.
Name: Shroomish
Availability: Petalburg Woods, 15%, Levels 5-6
Stats: Good bulk with low offenses and Speed. Upon evolution, its Attack stat skyrockets to become obscenely high, especially at such an early point in the game.
Typing: Grass typing is useful for the earlier portions of the game due to its advantage against Rock and Electric. Upon gaining the Fighting type upon evolution, it gains favorable matchups against Normal-, Dark-, and Steel-types, while possessing neat resistances to Ground, Water, Dark, and Rock.
Movepool: Shroomish relies on disruptive moves such as Stun Spore, Leech Seed, and Mega Drain to take full advantage of its bulk and make up for its lack of power. Once it evolves, it changes gears to become an attacking powerhouse, crushing foes with moves like Bulk Up, Mach Punch, Rock Tomb, Silk Scarf-boosted Headbutt / Strength, Sludge Bomb, and Sky Uppercut, most of which are very powerful attacks for when they are learned compared to most Pokemon.
Major Battles: Shroomish can solo Roxanne while being able to hinder Brawly with Leech Seed. Breloom can smash Wattson, Archie, and Sidney pretty easily as well as wipe out most of Norman's trainers, while Norman himself can be dealt with clever use of Bulk Up and / or Counter. It loses to Flannery, Winona, Tate&Liza, and Phoebe, but otherwise its immense power makes it a threat against nearly any opponent it fights, especially with the threat of Bulk Up + X-Speed sweeps.
Additional Comments: Breloom's usefulness against Glacia is heavily decided on whether Breloom is able to outspeed her Pokemon. Try to catch a Shroomish with a +Spe nature (or at least not a -Spe nature) and feed it any Carbos you can find.
Name: Absol
Availability: Route 120 Grass, 8%, Level 25 or 27
Stats: Glass cannon; very high Attack stat with decent Speed and Special Attack, but poor defenses.
Typing: Dark typing is useful for its Psychic immunity and Ghost resistance, but does not serve Absol very well in the attacking department at all.
Movepool: Absol is TM-reliant, but fortunately it has a very diverse movepool that can easily be tailored to your needs. It has Swords Dance to bolster moves such as Shadow Ball, Aerial Ace, and Strength to extremely powerful levels, or it can take advantage of the coverage granted by Ice Beam, Thunderbolt, and Flamethrower, which it can further augment with Calm Mind if it wants.
Major Battles: Ice Beam + Thunderbolt Absol should wipe out Winona's Gym provided it OHKOes every target. Absol can smash Tate&Liza, though it is concerned by Emerald Claydol's Earthquake and Xatu's Confuse Ray. Absol can use Calm Mind + Thunderbolt to beat Juan/Wallace provided Whiscash is out of the way first, which usually means setting up on the Sealeo or Juan's Crawdaunt. BoltBeam coverage or Swords Dance + Strength help combat Sidney, while Swords Dance + Shadow Ball should wipe out Phoebe (although her lead Dusclops's Curse will deny the clean sweep). Absol could attempt a Calm Mind sweep against Glacia and Drake, though Absol's fraility makes this an unpractical endeavor without using up a ton of items, and is usually better off only attempting to pick off one of their Pokemon at a time. The fraility is made even more apparent against the Champion, where Absol is not expected to boost very safely at all.