Generation VI obviously objectively has the best as far as details and animation go, but I actually voted for Generation II.
The way I see it is that it is really split into three categories, Generation I and II's "prehistoric" sprites that were very hardware limited, Generation III, IV, and V's sprites that finally could use all of the true colors they needed, and then Generation VI is in its own category with 3D models.
Generation I's sprites obviously had a lot of issues that Generation II's sprites fixed, but that gives them their own uniqueness as they are definitely the silliest of all 6 generations. But if you're like me and like your Pokemon sprites to...actually look like the Pokemon...which Gen I did not always do, especially when it came to backsprites, Generation II is the best.
As for the second group, I feel bad for Generation III because Generation IV was pretty much just a plain and simple straight upgrade from its sprites. Unlike I, there wasn't much silliness to the Gen III sprites to give it its own uniqueness either (Feraligatr ass is the exception). Generation V of course is the only generation to have full body sprites, which makes it very unique.
Gen VI is just awesome as we all know, there's really not much to say about it. I will say it does fail in some regards in respect to the flying Pokemon. All the Pokemon have to perpetually fly now due to sky battles and for some of them, like Charizard X, it looks bad. The issue is too much realism; in the previous games, they just had a little bit of color for a background and the angle of the battle always made the opponent look more elevated than your Pokemon anyway. Now that the backgrounds and settings for each battle are completely realized, and the Pokemon are more fully animated, it looks weird to see something like Charizard-X, Honchkrow, or Skarmory, rather than flapping their wings, in a constant gliding pose and staying in place.
P.S. I like how I said "there's not much to say about it" and it ended up being my longest paragraph.