One's favorite Pokémon will almost always depend on what you grew up with, practically every Pokémon fan will have gotten into it during their childhood, very few do so later than that, and as always nostalgia is something that's impossible to argue against. You also get a certain attachment to your long-time favorites just because you'll naturally have had many more experiences with them, like it doesn't matter what designs they pump out these days, they can't really compete with Pokémon that I've used in games for almost 20 years, because they have to start from 0 while there are a ton of Pokémon which are at 100 for me.
It doesn't matter whether you grew up with Gen 1, 2, 3, 4, etc, I think it's one of those series where it's perfectly fine to let nostalgia influence you, take that away and it loses a ton of its appeal. I think the Pokémon community would be a lot better if it just accepted that, practically everyone is affected by it anyways, hence why the "best game in the series" opinion cycles ever so predictably through each generation one by one.