6. Generation 6 (Pokémon X & Y)
The leap in quality between gen 5 and gen 6 is pretty steep. While Pokémon X and Y have their share of crap creatures (which we'll get to in a moment), it's nothing like the dark days of Pokémon Black and White.
Unfortunately, the trend of slapping faces on inanimate objects continues, with Klefki, an actual bunch of keys, being one of the worst offenders of the entire franchise. Elsewhere, Tyrantrum is a poor imitation of Tyranitar, and Binacle and Barbaracle are some of the worst designed Pokémon ever. Gen 6 also has the dubious honour of having the worst set of starter Pokémon, with only Froakie and its evolutions being half decent.
That said, there's plenty to love in gen 6. Xerneas and Yveltal are killer legendaries, Pancham has become one of the most famous creatures of later generations, and Gogoat has both an awesome design and impressive stats. The ghost types introduced in this game (Phantump, Trevenant, Pumpkaboo and Gourgeist) are some of the best in the whole series, and Vivillon, with its many different patterns dependent on where it is caught, makes for a fantastic collector's item.