here's the answer for your question with all the knowledge I have and with a bit of help with the video that MandJtv made aka micheal .
kanto gen 1 red/blue : bulbasaur, mainly because it one handedly handles brock then misty.after that heres the type match up lt.surge resists,erika resista,sabrina weak but wilk survive atleast one hit t counter,koga neutral,blaine weak,giovani strong
yellow : easy pikachu. because it is the only one.
gen 2 johto g/s/c :even though totodile does do a better with surf and strength but will lack power for e4. so cyndaquill the better starter. i've used it in silver and handled the ef alone almost had to switch here and there but mostly typhlosion handles everything with this moveset
fire pinch
thunder punch
earth quacke
strength
gen 3 hoen r/s/e : mudkip but torchic close second. handels roxanne,wattson,flanner,tate and liza
gen 3 kanto fr/lg : bulbasaur same as gen 1 r/b
gen 4 d/p/plat : chimchar roark,gardenia,maylene's lucario,candice,byron
gen 4 hg/ss : totodile you can figure it out by watching micheal's can I beat Pokemon hg without cathcing any Pokemon.
gen 5 b/w/b2/w2 :piginite brycen burgh lenora
gen 6 x/y : as you said delphox,venasaur
gen 6 oras : again mudkip for same resons as gen 3 but with mega
gen 7 s/m/us/um : poplio handles almost half of the trials alone and with a creative move set maybe even more than half.
gen 7 lp/le : depending on version eevee for let's go eevee and pikachu for let's go pikachu
gen 8 sw/sh : scorbunny because it handles milo,melony for shield but not for sword and raihan's ace duraludon
gen 8 bd/sp ( with all info we have) : chimcahr same resons as gen 4
gen 8 legends arceus : we don't know a lot about it so far but we know that the starters are cyndaquill,rowlet,oshwat.
hope this helps because i'm an kid (12 years old) so please don't scold me I tried my best to give the best answer.
sorry in advance if you didn't like it