Never played SSB64, but if I had the chance I’d go with Pikachu or Jigglypuff.
As for Melee, I started with Pikachu, but after I figured out how to unlock all of the characters and why the GameCube wasn’t saving the data, I switched to Mewtwo and Pichu.
In Brawl, which I’ve been replaying, I found that Falco is fun to use, although I’ve liked using Pokemon Trainer and Lucario.
In SSB4, I used Lucario until Mewtwo released. Mewtwo was my first favorite Pokemon for the incredibly shallow reason of “it strong.”
In Ultimate, I main whoever I used in every other game I’ve played, although I have taken a liking to Wolf. I never bothered to get into using him in Brawl because he was always unlocked really late, and I was too much of a wuss to get into the Subspace Emissary and unlock him faster. Yes, kid me was afraid of a button on the Singleplayer menu.
mains are kinda boring so i just play whoever is midly funny and doesnt have some sort of annoying ressource management, doesnt completely dominate our braindead 2v2 matches like pyra or min min, doesnt really require combos to have fun with and idk whoever i just like in some way. basically i split the roster in two between who i can always consider playing and who i pretty much always avoid
heres the roster i edited for like 10 min only to realize it doesnt even have all dlcs. its characters ive had at least some fun with in our arena matches while i avoid anyone greyed out
fellow toon link user 
