Idk much about RBY and GSC, but Psychic was the god-type back then and Starmie hits a pretty high speed so it should have been p good.
Don't talk about gens you don't understand.
Starmie's kinda an oddball in RBY. On one hand, it beats most of the meta thanks to its Ice/Psychic resists plus Thunder Wave and Recover combined with a wide selection of special attacks (Blizzard, STAB Surf, STAB Hydro Pump, STAB Psychic, Thunderbolt). On the other hand, its Electric weakness means Chansey can kill it, which means it's walled (while Alakazam's not, because it can outstall Chansey), and it doesn't really like tangling with Alakazam itself either for complicated and somewhat recursive reasons. The result is a defensive 'mon that usually runs from other defensive 'mons, but sweeps with little difficulty if they're removed. Also a really good way to lure and/or paralyse Chansey if you want to.
In GSC it's not nearly as good, mostly because it lures the newly-buffed Snorlax, which is on every team and not something you want to let in for free. With the Blizzard nerf and the introduction of RestTalking Electrics, those are a problem too. As such, it's more of a role-player than some sort of sweeping behemoth. Specifically, it's a highly durable and highly effective spinner (outspeeds Cloyster, Gengar dies to Psychic) and also one of the few real counters to Machamp, while also screwing with Charizard and Grounds like any Water.
Zapdos on the other hand: Great in RBY and GSC (apparently), great in DPP, not as great in ADV and UU in BW
hurr
Zapdos in RBY is decent, but only decent. Thunder doesn't have a 30% paralysis chance yet, Hidden Power doesn't exist yet, Sleep Talk doesn't exist yet, and Blizzard is really fucking common. What this translates to is a Pokemon that hits hard from two uncommon and useful STABs (super-effective STAB on Waters AND Exeggutor is awesome, and non-Normal physical STAB allows it to hit Chansey and Gengar), but has hard walls in the Rock/Grounds, needs Thunder Wave if it's to spread status, and has a hard time switching in on most things (in particular, it does NOT counter Starmie or Lapras). Its Speed is also really awkward; the common stuff in RBY is either really slow (base speed 55 - Exeggutor - or slower; I guess there's Lapras at base 60) or really fast (base speed 110 - Tauros and Gengar - or faster). So base 100 isn't quite enough.
Zapdos in GSC is amazing, second only to Snorlax. Status sponge extraordinaire, immune to Spikes, can actually beat the Grounds with Hidden Power. Snorlax and Raikou are really the only things that can deal with it (Blissey is bad in GSC).
From what I understand of ADV ADV!Zapdos > RBY!Zapdos.
The difference is they share the same dex number. They are the same species.
As I said, terminology. Chansey and Blissey are the same species in any context where biological terms are used somewhat properly, as opposed to Pokemon's hilariously bad misuse.
Why are we talking about Rotom formes when nobody believes it to be top 10 regardless
Because Chansey/Blissey is a candidate if you count them as one 'mon, which they basically are.