I'm not exactly searching for the worst Pokémon in terms of pure battle prowess. The answer to this one might be a lot clearer with things like Unown existing (which for the sake of the argument i'll exclude from this discussion since even in the grand scheme of things it's still Unown)
The Pokémon i'm looking for is a Pokémon that
>Has consistently limited availability (Late game, Low % rate, Convoluted evolution, anything that can deter a guide-less person from using it)
>Is more or less outclassed not just overall but in the context of the games where it is available, in any stage of the game (As some Pokémon that kind of suck overall do shine in the early game because they're relatively strong for the early game)
Among other traits that would make a Pokémon only appear in a given trainer's team if that trainer actively wished for that Pokémon to be there, and it took an extra level of effort for the trainer's enjoyment, rather than performance.