Hard to choose from the many terrible gen 5 Pokémon. I've narrowed it down to Gothita, Gothorita, Gothitelle, Solosis, Duosion, Reuniclus, Sawk, Throh, Landorus, Thundurus, the other -orus whose name escapes me, Emboar, Timburr, Gurdurr, Conkeldurr, Tympole, Basculin, Maractus, Scraggy, Scrafty, Trubbish, Garbodor, Minccino, Cinccino, Emolga, Karrablast, Escavalier, Vanillite, Vanillish, Vanilluxe, Foongus, Amoonguss, Klink, Klang, Klinklang, Elgyem, Beheeyem, Accelgor, Mienfoo, Mienshao, Durant and Kyurem.
And if I had to narrow it down to one, it'd probably be Emolga for being a blatant Pikachu/Pachirisu cute little electric type rodent cash-in, and for being almost impossible to run from in the wild for some reason.