I just beat the E4 on virtual console Crystal with this team:
Meganium (body slam, razorleaf, psnpowder, reflect) lv.38
Poliwrath (surf, ice punch, hypnosis, submission) lv.39
Ursaring (strength, fire punch, thunderpunch, dig) lv.38
Fearow [KENYA] (peck, swift, mirror move, fly) lv. 38
Espeon (psybeam, shadow ball, bite, quick attack) lv. 39
Magmar (sunny day, dizzy punch, thunderpunch, fire punch) lv. 38
I believe using six Pokemon revealed their advantages and disadvantages better than a solo/duo/trio run could since being underleveled a bit for the E4 is a good test of a Pokemon's abilities, though I am aware of the typical party of three assumption. I barely had to grind to make it up through the E4, though I did casually dilly-dally (read: "using up" in-game time) at times since I hadn't played through GSC in so long. Each base Pokemon was obtained before Morty and not once did any of them leave the party after, or even hold the Exp. Share. No HM slaves, so the move deleter and Poliwrath were good friends to say the least.
Poliwrath is really great in Crystal and very glad I stumbled upon him early. There's been some discussion between Poliwrath (C) vs Quagsire, which has been illustrated by some other users. If anything, I think they should be in the same tier. If you have Crystal I really don't think you can go wrong with either of them, and along with Psyduck I think they would all get along together in A since they are all very solid water types that play a bit differently from each other. Poliwrath's bulk, versatility, and earliest water-type availability (besides Toto obv.) raises it above the other B water mons in my opinion.
Finding Poliwag almost first thing on Route 30 in the grass (Crystal only) gives it an upper leg when it comes to availability, and learns hypnosis and water gun fairly quickly at lv. 7 and 12 respectively. Headbutt is nice too and early once you get to Goldenrod. Hypnosis puts Poliwag a step above other water mons because it is bulky enough to afford to miss with it occasionally, and Poliwhirl, despite it's brief awkwardness, likes abusing it with its base 90 speed. Realistically you can put almost anything out of commission temporarily and blast away with surf, or switch something else in. It is a great team support Pokemon in that regard, and counters pretty much every fire type (Poliwrath's fighting type does not like Charizard though) and more notably Karen's Umbreon and Houndoom. Ice punch is indispensable of course. Getting a water stone from Fisherman Tully is trivial in crystal if you have just his number. This early availability of the water stone should be enough to separate Poliwrath (GS) and Poliwrath (C), which may need to include a discussion since there are other stone-sensitive Pokemon like Victreebel that appear to be greatly improved in usage. Poliwrath's fighting type puts him out against flying and psychic types unfortunately, but does help against random rock and dark moves. I wasn't able to get Politoed, so maybe that deserves another look as well, though I think Poliwrath's fighting type and increased defense does give him a slight advantage. Submission at least helped with Umbreon, all Politoed gets is gimmicky perish song.
Kenya the Spearow is a well-known superstar, and I think Kenya deserves his spot in S. However, I barely had to use him since he grew like a weed due to the exp. boost, which leads me to question non-Kenya Spearows, though I think they're advantage over the Sprout Tower and Bugsy is a plus. I'm fine with it staying in S because even if I did not reveal it's true potential in drill peck by the E4 it's still a great Pokemon in-game. Also mirror move is basically useless and wasn't relevant for me in fights such as Clair. Also fly.
Yeah, we already know Meganium is severely overshadowed by the other starters, however I wouldn't say it's a terrible Pokemon to choose. Reflect did come in handy at times, notably against Lance to aid Poliwrath and itself against Hyperbeams/Wing attacks, and had no trouble at all with pretty much any water type except for maybe the Dewgongs in Pryce's gym and Lance's Gyarados. C tier may be a little harsh, but it really is restricted to being a good support mon, so of course it thrives best in an even party of 6. I can't imagine it being too helpful with a smaller team and I would rather go for Bellsprout (C) if I wanted a more offensive type, but the pure grass type on Chikorita does come in handy against psychic-type moves. I think C tier is a fair assessment as a Pokemon, but the fact that it's a starter does help it if you don't want to opt for the other two. It's not the most efficient Pokemon and proved little service for the first couple gyms, but it's not the most inefficient either. Mid-tier is not terrible though but there are clearly better options if you're looking for power efficiency. Even a different supporting grass type such as Bellossum seems almost as good. Poison powder is lol and I am not really sure why I skipped Synthesis and kept it, don't think it would change much though.
Ursaring (C) is A-tier no question. Bulky, powerful, very early, can learn the punches and make good use of strength HM. No debate here, even if it does have a low-encounter rate and a flee-factor. Belongs beside the other normal powerhouses Tauros and Miltank.
Espeon is a strange case for a few reasons, so I do not think it should rise above B at all. I was stuck with Eevee until after the 8th badge, though Eevee isn't too bad with Headbutt, and I also did sacrifice my shadow ball TM but it was worth it because it would have probably been dead-weight before bite at lv. 30. Good thing normal types prevail in the early game, though. The friendship thing is a different issue, since I don't think there's really an efficient way to get it to evolve before Espeon learns Psybeam at lv. 36 without pumping a lot of money into vitamins or using X-items at every possibility. I beat the E4 in 3 days so I only got it 2 haircuts, but it's possible to change the 3DS clock but I didn't think it was worth that much trouble just to get a few extra haircuts. Espeon with bite and psybeam helped rip Will and Bruno apart, but the friendship slog by pretending to like your Eevee keeps it out of A despite its wonderful stats. It is not on the same level as the immediately useful mons in A.
Now, the odd egg is a controversial topic to begin with, but Magby was extremely valuable. E-tier is a joke. How is Pichu (C) in D-tier and Tyrogue (C) in the C-tier! To me, Magby is clearly the best odd egg Pokemon and if you are lucky enough to get it I highly recommend using it. I think it should rise up to C-tier. My odd egg hatched right after grabbing Eevee from Bill, so hatching doesn't take too long if you pick it up first thing after Ilex Forest. It caught up in no time and proved useful for Jasmine's Magnemites. It does face very strong competition against Growlithe for catchable fire types in Crystal.
First, it is the most offensively stable baby of the bunch. 75/70/83 at/spatk/spe spread was able to assist it to lv. 30 before evolving. The dizzy-punch comes in handy, but learning thunderpunch gives it nice coverage. This little guy was ballsy enough to occasionally handle water pokemon with its speed and decent special attack. Magmar (GS) is A-tier, but in G/S you can find it as a Magmar much earlier than Magby evolves, and the odd egg factor with 0 DV's across the board does not help Magby's case. Magmar (GS) is obviously way better than Magby (C) and is catchable at the same spot in G/S that my Magby hatched, but I think Magby belongs in D-tier at the absolute minimum. Tyrogue should either be brought down to D from C or Magby (C) should be brought up to C. Magmar is a seriously useful Pokemon, but the odd egg factor definitely caps it at C. I have no idea about Smoochum or Elekid, but I don't really see how Pichu is above them. I admit that Tyrogue is advantaged for lv. 20 evolution, but it is awful before that and literally must be babied I would think. Magby carried its own despite not being fully evolved yet. Also Magmar seems way more useful than any of the Hitmons.
Since I made a mistake using the shadow ball TM on Eevee, unfortunately I don't think Celebi will be very helpful even if it would perfectly replace Meganium. Shadow ball appears to be Celebi's main move before Giga drain and Psychic, and I wouldn't want to rely solely on Solar beam or getting a decent Hidden power. I think I was reading this thread while playing and found out here that Celebi is available in the virtual console version... after using the shadow ball TM... oh well! That is why I'm going to try out either Larvitar or a lv. 40 Dragonair/Dratini caught with a super rod to assist with Red. The only other decent Pokemon that I could get exclusively in Kanto is Aerodactyl, but honestly he seems to belong in E. His movepool and stats are not bad, but having to catch a Chansey at a 1% encounter rate gives him little leverage to pursue especially so late. Though there are probably plenty of things in the E-tier that it is better than.
I envision Larvitar being able to at least join Dratini in the D-tier judging from the post of newseeds. I will try to report back after beating Red.
TL;DR for pre-Kanto tiers
Meganium -> fine in C (or whatever mid-tier that may be created)
Kenya the Spearow -> fine in S
Poliwrath (C) B->A
Espeon -> fine in B, no higher
Magby (C) E->D or even C... deserves a little more respect!