EV Training Spots in Black/White
 

Just in case anyone else was thinking about it! Thanks to Phiddlesticks for helping with this. If anyone has any better ideas for Def training, or you can confirm how many Atk EVs Druddigon gives, feel free to post here.

HP
Surf in the dark puddles of Icirrus City - Stunfisk (100%, 2 HP)

Also, Victini (3 HP) can be rebattled infinitely as long as you do not catch it.

Atk
Route 1 - Lillipup (50%, 1 Atk) and Patrat (50%, 1 Atk)
Dragonspiral Tower 2F - Golett (100%, 1 Atk)
Dragonspiral Tower 1F - Golett (50%, 1 Atk), Druddigon (30%, 2 Atk), Mienfoo (20%, 1 Atk)
Outside of Dragonspiral Tower, Double Grass - Tranquill (30%, 2 Atk), Sawsbuck (30%, 2 Atk), Mienfoo (30%, 1 Atk), Druddigon (10%, 2 Atk)

A couple of notes need to be made here.
- During Winter, the Double Grass location changes up. Druddigon is replaced by Beartic (10%, 2 Atk), which is great, but Tranquill is replaced by Vanillish (30%, 2 SpAtk).

Which of these locations is the best is up to you. If you don't mind keeping track exactly how many EVs you got, Dragonspiral 1F or outside Dragonspiral is the place for you. Outside gives the most EVs at in a short time, especially if you can OHKO two enemies at a time. If you'd rather keep it simple, Dragonspiral 2F or Route 1 are where it's at.

Def
Wellspring Cave - Roggengola (50%, 1 Def)
Pinwheel Forest - Venipede (15%, 1 Def) and Sewaddle (35%, 1 Def)
Pinwheel Forest Double Grass - Whirlipede (15%, 2 Def) and Swadloon (35%, 2 Def)
Chargestone Cave - Klink (29%, 1 Def), Ferroseed (20%, 1 Def), Boldore (10%, 1 Atk, 1 Def)

None of these guarantee a Def encounter. Wellspring Cave also has 50% Woobat, Pinwheel has Cottonee/Petilil (35%) and Pidove (15%), Pinwheel Double has Cottonee/Petilil (35%) and Tranquil (15%), and Chargestone Cave has Joltik (36-39%) and Tynamo (2-8%), depending which floor you're on. Note that Boldore gives 1 Atk in addition to 1 Def!

SpAtk
Celestial Tower - Litwick (100%, 1 SpAtk)
You can go to higher floors if you want, those have Elgyem (15%) which also give 1 SpAtk. There's a Nurse on 3F, which is convenient.

SpDef
Surf in Driftveil City - Frillish (100%, 1 SpDef)

Speed
Surf on Route 3 - Basculin (100%, 2 Speed)
You can Surf in a lot of places for 100% Basculin, but this is an example of a place where you don't have to go through tall grass or anything to get to a surfing spot when coming from a close Poké Center.

This definitely needs to be stickied. Thanks Mekkah!

Also just a note: Stunfisk is found in the puddles of Icirrus.

For Defense, Victory Road has Durant (40%) and Boldore (20%), both 2 Defense EVs.

Also, there is Black City, but its quite slow IIRC. I haven't played Black yet, so I'm not sure. :/

20% encounter rate, though. Surf in the dark parts of those puddles for 100% encounter rate.

CWW, both bulbapedia and serebii say that Boldore gives 1 Atk and 1 Def EV :(

For Attack you can also choose to train in Dragonspiral Tower instead. Druddigon(Cream Gun) has a healthy 3 ATK EV payout.

druddigon gives 2 atk EVs.

Wait, Phiddlesticks is on here? Whoa. Blew my mind. I don't think I've ever seen a post by him.

i pretty much just lurk the forums, im only really active on irc

also, serebii says druddigon gives 2 attack evs, bulbapedia says it gives 3. oof!! also druddigon isnt very common inside dragonspiral tower, but luckily the goletts and mienfoos that can also be found give attack evs as well. basically, inside dragonspiral tower is much better than route 1, good find tm13

but an even better place than inside dragonspiral tower is the dark grass just outside of dragonspiral tower, where you only fight tranquill (30%, 2 attack), sawsbuck (30%, 2 attack), mienfoo (30%, 1 attack), and druddigon (10%, 2 or 3 attack). only problem is that in the winter the tranquills get replaced with vanillish, who gives 2 spatk.

Is there some common pokemon that give just 1 HP/ 1 speed EV?

For those with Victini, it gives 3 HP evs and is easily rebattled. You may want to catch it though, lol

duckletts on the driftveil drawbridge give 1 HP ev, but thats probably a lot more tedious than stunfisks. if you just need it for a few points to finish off a stat it should work fine though

for speed, theres woobats in the mistveil cave (50% chance) who give 1 EV each, but again, a lot more tedious than basculins

Updated with suggestions up to here.

The relic castle makes an excellent location for defense training. Yamasks appear on the first floor 50% of the time and award 1 defense point a piece.

The last few levels also have Sandslash, Cofagrious, and Onix, which give 2, 2, and 1 Defense points respectively.

Gamefreak Morimoto gives 12 Spd evs and can be battled once every day.

Black City maybe? (as stated before) Some are quite high level so you'd have to do so preparations before training there. You can rebattle every trainer instantly, each has one Pokemon.

FYI. I have an EV Manual already written for 5th Gen in the Articles section.

I found a defense spot! 89% chace of encounter.

a bit of a pain in the arse though...

route 11 + level 50 dead poke first in party + repel = 25% vullaby and 15% Gligar. Run from 5% Poniards.

(25+15)/(25+15+5)=0.88888888

also black only fffff

For SpDef, the Volcarona Room is a nice place (100% Claydol, 2 SpDef).

Very high levels, though.

For the record is it even possible to access the Doubles Grass outside of Winter? I have no idea how to do it otherwise. >_>

No, just use the 100% golett in Dragonspiral

(and yes you do do it in winter)

why bother with 100% Golett when you have a slight chance of getting even more from Druddigon?

Because most of us want consistency; I measure how much EVs my Pokemon has gotten by the PP I've used so far (i.e. 25 of one move is 250 EVs).