Horde encounters were a new feature added in Pokémon X and Y, and one of their (perhaps unintended) benefits was to make EV training a whole lot easier. It's possible to get as many as 60 EV's from a single horde battle, and with the EXP Share on you can net as many as 360 distributed across the entire team.

Horde training is a little more of a headache in XY than in ORAS because there are fewer locations that give only EV's of a single type, and those that give EV's of multiple types often have weird combinations that aren't very useful for training. Below are your best options for training each single stat, including how often you can expect to find a horde giving that type of EV in that location. Click the location's name to go to a full description of its hordes.

These are the full horde encounter tables for each route, as well as how many EV's each different kind of horde gives:

XY Hordes by Route
Pokémon		Level	Encounter Chance	Base EV Yield
Connecting Cave (HP, Attack, Speed)
	Whismur	7	60%	5 HP
	Zubat	8	35%	5 Speed
	Axew	8	5%	5 Attack
Reflection Cave (Defense, Special Defense)
	Mime Jr.	11	60%	5 Special Defense
	Roggenrola	11	35%	5 Defense
	Roggenrola (4)/Carbink (1)	11	5%	5 Defense, 1 Special Defense
Azure Bay (HP, Defense, Speed)
	Wingull	13	60%	5 Speed
	Slowpoke	13	35%	5 HP
	Exeggcute	14	5%	5 Defense
Frost Cavern (Attack, Special Attack)
	Vanillite	20	60%	5 Special Attack
	Cubchoo	20	35%	5 Attack
	Smoochum	21	5%	5 Special Attack
Terminus Cave (Attack, Defense)
	Durant	23	60%	10 Defense
	Geodude	23	35%	5 Defense
	Aron (X)	24	5%	5 Defense
	Larvitar (Y)	24	5%	5 Attack
Pokémon Village (HP, Special Defense, Speed)
	Foongus	25	60%	5 HP
	Poliwag	25	35%	5 Speed
	Lombre	25	5%	10 Special Defense
Victory Road, First and Last Caves (HP, Defense, Speed)
	Geodude	28	60%	5 Defense
	Floatzel	29	35%	10 Speed
	Lickitung	30	5%	10 HP
Victory Road, Second and Third Caves (HP, Defense)
	Geodude	28	60%	5 Defense
	Graveler	29	35%	10 Defense
	Lickitung	30	5%	10 HP
Route 5 (HP, Attack, Speed)
	Gulpin	5	60%	5 HP
	Scraggy	5	35%	5 Attack
	Plusle (4)/Minun (1) (X)	6	5%	5 Speed
	Minun (4)/Plusle (1) (Y)	6	5%	5 Speed
Route 7 (Special Attack, Special Defense)
	Hoppip	7	60%	5 Special Defense
	Psyduck	7	35%	5 Special Attack
	Roselia	8	5%	10 Special Attack
Route 8 (Attack, Special Attack, Speed)
	Wingull	7	60%	5 Speed
	Zangoose (4)/Seviper (1) (X)	8	35%	9 Attack, 1 Special Attack
	Seviper (4)/Zangoose (1) (Y)	8	35%	6 Attack, 4 Special Attack
	Taillow	7	5%	5 Speed
Route 10 (Defense, Special Attack, Speed)
	Nosepass	10	60%	5 Defense
	Yanma	10	35%	5 Speed
	Houndour (X)	11	5%	5 Special Attack
	Electrike (Y)	11	5%	5 Speed
Route 11 (HP, Speed, Attack)
	Nidoran♂ (4)/Nidoran♀ (1)	11	60%	1 HP, 4 Attack
	Stunky	11	35%	5 Speed
	Starly	12	5%	5 Speed
Route 12 (Attack, Defense, Speed, Special Attack)
	Wingull	13	60%	5 Speed
	Mareep	13	35%	5 Special Attack
	Tauros (4)/Miltank (1)	14	5%	4 Attack, 2 Defense, 4 Speed
Route 14 (Attack, Defense)
	Bellsprout	16	60%	5 Attack
	Skorupi	16	35%	5 Defense
	Ekans	17	5%	5 Attack
Route 15 & 16 (HP, Defense, Speed)
	Murkrow	18	60%	5 Speed
	Foongus	18	35%	5 HP
	Klefki	19	5%	5 Defense
Route 18 (Defense, Special Attack)
	Geodude	23	60%	5 Defense
	Durant	23	35%	10 Defense
	Durant (4)/Heatmor (1)	23	5%	8 Defense, 2 Special Attack
Route 19 (Attack, Defense)
	Weepinbell	24	60%	10 Attack
	Gligar	24	35%	5 Defense
	Arbok	25	5%	10 Attack
Route 20 (HP, Attack, Defense)
	Foongus	25	60%	5 HP
	Trevenant	25	35%	10 Attack
	Trevenant (4)/Sudowoodo (1)	25	5%	8 Attack, 2 Defense
Route 21 (Attack, Special Attack, Special Defense)
	Spinda	26	60%	5 Special Attack
	Swablu	26	35%	5 Special Defense
	Scyther	27	5%	5 Attack