In Gen VI, EV training has become significantly easier thanks to horde encounters. Adding Pokérus and Power items, it's possible to raise a specific stat by as much as 60 EVs per battle. Sweet Scent always triggers horde encounters unless there is a wheather condition (possibly only in rain, I'm not sure there).
Since EVs can rise so fast with this method, it's now also relevant how far these spots are from each other or from the nearest Pokémon Center, so that the total time used to fully EV train one or several Pokémon can be minimized.
So now the question is where the best spots for efficient horde EV training are in X/Y.
- The number of different Pokémon that can appear in hordes at the specific spot and their respective EV yields
- If there are mixed hordes at the specified spot, the different Pokémon's EV yields
- Proximity to other spots and to Pokémon Centers to minimize the time to get from spot to spot
An ideal spot would have only hordes in which all Pokémon give either +1 or +2 for a specific EV (same for all different hordes in that spot) and be reachable from a Pokémon Center very close to the city entrance.