Out of the seven generations that currently exist, which one has (in your opinion) the best collection of new Pokémon? Maybe it's because it holds many of your favorites, or because you just can't think of any Pokémon in it that you actively dislike.
Pokémon BW2 had a really great Pokédex. They took the dex from BW and added several new Pokémon to it which greatly enhanced the game, such as the Riolu line, Eevee, Arcanine, Electivire, etc. I thought the collection was great overall, and I have some of my most memorable teams from that dex.