What are the best places to horde EV train?

Question

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.
Criteria are:

• 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.

Answer 1

This Smogon Guide to EV training with Hordes should be exactly what you're looking for. As for the 'best' places, here's what they suggest:

• HP - Route 5 (Gulpin), Connecting Cave (Whismur), Route 20 (Foongus)
• Attack - Route 19 (Weepinbell), Route 14 (Bellsprout)
• Defense - Terminus Cave (Durant, Geodude or Aron), Victory Road (Geodude, Graveler)
• Special Attack - Frost Cavern (Vanillite or Smoochum)
• Special Defense - Reflection Cave (Mime Jr.)
• Speed - Route 8 (Wingull, Taillow), Route 12 (Wingull), Route 15 (Murkrow)

These routes take into account frequency of encounters, number of EVs gained per Pokémon defeated, and ease of access to the location.

To address your specific criteria:

• The number of different Pokémon that can appear in hordes at the specific spot and their respective EV yields

For all the best areas listed above, the Horde encounter is the 'Common' one. You may still need to run from a different type of horde every now and then.

• If there are mixed hordes at the specified spot, the different Pokémon's EV yields.

I'm going to point you to the Horde Guide again. This lists all available hordes in all areas of the game and give their respective EVs - including the mixed ones.

• Proximity to other spots and to Pokémon Centers to minimize the time to get from spot to spot

I'm not sure what to say here. Most routes connect directly to towns with Pokemon Centres, and have grass almost straight away. The most annoying I've come across is Route 8 Wingulls & Taillows - If you want to walk back to the Pokemon Center you have to complete the rock-hopping challenge again. But this and any other walking-difficulty can be averted entirely by keeping a Flying Pokemon in your party.

Note: I'd also like to add that personally, I found EV training Attack on Routes 19 & 14 to be really annoying due to the seemingly almost constant rain cancelling out Sweet Scent. As an alternative, I suggest farming Trevenant in the Winding Woods - yes it is an 'Uncommon' encounter, but it seems to appear almost 40% of the time for me anyway. Just watch out for the Rare Trevenant + Sudowoodo hordes. Sudowoodo gives Defensive EVs.