I recently completed a run of this game and saw some of the things I used being discussed here. My team was picked by friends and I used Torchic/Lotad/Geodude/Shedinja/Swablu/Voltorb.
Torchic: I disagree with WaterBomb's assessment and I think keeping it S-Rank is fine. I found that using Brawly's Bulk Up TM immediately on Combusken made up for the mid-game issues he described, and I was actually able to use Combusken to solo Flannery and Norman with Bulk Up and a few potions. It does run into some Speed issues against Norman but iirc the Petalburg Mart sells X Speeds so that easily makes up for it. Blaziken's offense is fine on the water routes if given the Return TM from Cozmo, since by the time Lilycove is reached it should definitely have maxed out friendship and 102 BP Return off 120 base Attack is gonna shred Wingull/Tenta lines pretty easily, especially since the majority of the swimmers on the sea routes use NFEs iirc. The only fights I found Blaziken wasn't able to contribute were Tate and Liza, Drake, and Wallace - more or less everything else falls to it.
(Glacia is definitely a plus MU in practice fwiw - Blaziken can set up 1-2 Bulk Ups on her lead Glalie and then Brick Break should KO everything)
I would also argue that RS!Torchic definitely shouldn't drop, since unlike Wallace Blaziken has a great matchup against Steven. and Tate and Liza is much easier to defeat in RS as well.
Lotad: I made the first version of this tier list and I had ranked Lotad highly on it. After using it again I can conclusively say that I didn't know what I was thinking at the time and I profusely apologize. Lotad is pretty bad. It's good early on because of a fast first evolution and a strong matchup against Roxanne, but after that it just sucks sooooooo much afterward since its stuck with Bullet Seed and Nature Power as its only attacking moves until you get the Surf HM and Ice Beam TM after Gym 5. In fact, it doesn't get any Water moves at all until Surf, which prevents it from being able to contribute to Flannery despite being a Water-type. It does improve and become surprisingly workable once it gets Surf and Ice Beam, but then you have to contend with Lombre having pretty bad stats since the Water Stone isn't obtainable until after Gym 7, and being a Grass-type means it can't really contribute well against Winona even if given Ice Beam. Fortunately it does redeem itself as Ludicolo after Dive, since then you can also get Rain Dance TM and run a Swift Swim sweeper set that's pretty effective against Juan and most of the Elite Four. I don't think it's quite bad enough to be D rank but I also don't think its good enough to be C Rank either b/c of how low its offense is between Gyms 2 and 5 (a large segment of the game) - if there was a rank between C and D I would probably put Lotad there.
Geodude (Golem): Golem is good for a very specific segment of the game and that is Gyms 3-6 (having excellent matchups against all of them) and Magma/Aqua Hideout. It falls off hard after Aqua Hideout, maybe it was just me but my Golem was -Speed so I often saw it get outsped and killed by random trainer Water mons it probably shouldn't have died to. It's also not particularly useful in any of the Elite Four matchups, with a special shoutout for being completely useless against Wallace unless you want to try your luck with Quick Claw Explosion. Still, Golem's midgame dominance was nice for my particular team since Lotad was a late bloomer and Combusken sometimes needed backup against birds. It's good against enough of the game to where I think keeping it C Rank is fine, with the caveat that you shouldn't expect too much more out of it post Aqua Hideout.
Nincada (Shedinja): I would actually like to make an argument for Nincada to drop to E Rank. Leveling up Nincada to Level 20 is an incredibly frustrating experience. First, Nincada has no moves stronger than Scratch and Leech Life until L14 (unSTAB) Fury Swipes, and that is all it has until evolution where it learns...Fury Cutter. Smashing. This means that Nincada basically can't 2HKO anything unless its Poochyena, so as a result it kills things very slowly. Secondly, Nincada is in the Erratic EXP group. While Erratic requires the least amount of EXP overall, it requires the most EXP to level in the early levels by far, which means that Nincada kills things really slowly AND levels really slowly. This means that this mon requires an insane amount of grinding early game in the worst EXP group for early level grinding which is very time-consuming. And to add insult to injury, the end result is not worth it at all. Nincada and its evolutions have zero good boss matchups, especially Shedinja because it turns out Fire/Flying/Ghost/Dark/Rock coverage is not hard to come by on bosses in multiple instances. Ninjask AT LEAST has SD and Baton Pass and a Speed stat to redeem itself slightly, Shedinja however is slow and dies to a larger variety of Pokemon than you would expect. I only got use out of it against exactly two Pokemon and that was Juan's and Drake's Kingdras - I probably could have saved hours of my life if I had just Master Balled Rayquaza instead to do the exact same thing.
The only positive thing I can say about Shedinja is that it doesn't really need EXP anymore once it reaches Level 20 - its matchups are so binary that all you really have to do to ensure it wins its sparse winnable matchups is to give it the Toxic TM. However, that doesn't make up for the sheer pain spent grinding it early game only to have next to no payoff later. Using Shedinja is pretty much the exact opposite of efficiency and it needs to go rot in E Rank where it belongs.
I would also argue Ninjask to E Rank for similar reasons, though it does have SD, a Speed stat, >1 HP and later Baton Pass, so Ninjask at least has some team support perks that might make the slog worth it lategame. I don't personally think it is, but since I haven't used Ninjask myself I won't comment further on it.
Swablu: Swablu has a similar problem to Nincada - its in the Erratic EXP group, which means leveling it to Altaria is a huge pain in the butt, though it can at least be immediately TM'd Secret Power and Steel Wing to have some decent moves so its not as much of a painful experience. Swablu's raw offense is fairly mediocre but it can at least 2HKO things its own level with Secret Power/Return once it gets some EXP under its belt, so its an ok route sweeper. Altaria is a significant improvement at least - although its raw offense is still pretty lame, its typing is solid for handling the ocean routes, it has a lot of coverage options through TMs, and it does get the coveted Dragon Dance which gives it sweeping potential against some of the lategame bosses, particularly against Juan and Wallace (it can set up on his Whiscash). Aside from Wallace though its Ice/Dragon weaknesses keep it from being too effective against the Elite Four, and it doesn't have any particularly strong matchups against any other Gyms. Basically, it's ok - it gets the job done but there are better choices. C Rank fits it I think, I personally consider it very slightly below C Rank because its EXP group just sucks so hard when you first get it.
Voltorb: Voltorb was the biggest surprise for me and I'm going to argue for it to rise. It's currently languishing in D Rank but its honestly a lot more useful than that. It is captured at around Level 25-26 in New Mauville after Gym 5 which is fairly close to evolution, and it can be TM'd Thunderbolt immediately and unleashed on the ocean routes around Dewford to efficiently evolve (I assume you're going to visit the Abandoned Ship b/c a free Ice Beam TM is too good to pass up). Electrode statwise is a faster but weaker Manectric, but it still has plenty enough power to slay any fish and birds which there are plenty of after Gym 5. Now Electrode doesn't have any coverage options other than Thief or a lucky Hidden Power, which is somewhat unfortunate, but Manectric doesn't really do much better there with just unSTAB Bite. Where Electrode does shine over Manectric though is the additional support options it gets. Notably, it learns Light Screen and the majority of the remaining bosses after Winona (where Electrode has a great matchup) are primarily special attacking types (esp relevant for Tate and Liza, Juan, Glacia, Drake to an extent, and Wallace), or it can use Explosion so even in a bad matchup it can at least trade its life to kill or put a massive dent in a troublesome foe. I even took out Wallace's Milotic in my run by Exploding on it and then finishing it off with Blaziken. Now granted, Manectric does have more power and 2 Gyms more availability over Electrode - both are pretty significant ngl - but once Voltorb is available I felt that their performance was comparable, and it certainly wasn't the performance I would expect from a D Rank Pokemon.
Another small but important difference is that Voltorb is in the Medium Fast EXP group while Manectric is in the Slow EXP group, so while Voltorb will start off weaker than a leveled Manectric when it becomes available it will catch up pretty fast.
In short, I think Voltorb is worthy of C Rank. It's about as good at being a Thunderbolt machine as Manectric post Gym 5 and the unique options it gets over Manectric allows it to contribute more to matchups where Thunderbolt spam won't carry the day.
tl;dr Keep Torchic in S, Drop Nincada (at least Shedinja) to E, Raise Voltorb to C