My listed most useless Pokemon wins this; no doubt. At least most of the other Pokemon are still useful, while this one is just an abomination to the Pokemon World.
Wobbuffet... especially the female variety...
I know I like pink, but Wobbuffet is a stern pile of trash that just shouldn't have been turned pink, for its Shiny counterpart... I hate you for that, Game Freak... >_<
To look at the horrible thing, go check Bulbapedia, or wherever else has it shown, as a Shiny... I don't even want to think about it... >_<
Edit @ Shiny Questioners: The term "shiny" doesn't have to refer to the direct colour; usually just means shiny, as in a surface that is shiny, shich can be any colour. Pokemon Shinies refer to a rare alteration in colour; not the stereotypical reference to silver, whenever stereotypically talking about shiny. Plus, it would look pretty stupid, if every single shiny Pokemon was silver.
At least, in my opinion.