Right this is a very interesting one actually, I do think pichu and raichu do not not get enough love but they are some easy picks
Before pikachu was chosen, clefairy was actually the mascot! I believe you even see red with a clefairy on some early Manga, the reason they chose pikachu and he was so sucessful was that for the late 90's Pikachu was cute enough for the girls and cool enough for the boys! Whereas clefairy was more preferred by females
So using that concept considering this is in the late 90's I would actually pick Riolu. Riolu /Lucario are one of the few post gen 1 pokemon to break the main stream and make it to smash bro's, highlighting their popularity!
I pick riolu because it's cude and cuddly which girls in the 90's might like but also it's got a cool look and very cool fighting moves which the guys might like. Riolu could very easily struggle through some battle's in the anime, you could easily have it replicate the scene where pikachu gets destroyed by spearows, which really lets the viewer get emotionally attached to the pokemon! Riolu has a very distinct look which would like great on book covers, t shirts, trading cards, it's also got a golden shiny form
Riolu also evolves into Lucario which is so popular it's starred in one of the movies! Aura Sphere is a very cool signature attack which i think could be similar to thunderbolt.
Riolu/Lucario is also human shaped which would allow people to connect to the pokemon easier and it's easier for the pokemon to show emotion which i feel helps build a strong connection to the franchise
