First one. (i.e. 12 EVs)
Long Answer (accompanied by some maths and a formula from Smogon)
The fastest EV training is done with a combination of Pokerus and a Power Item, earning EVs at this rate per battle:
(y + 4) * 2
where y is the amount of EVs you would earn alone.
Metapod and Kakuna give out 2 EVs each. In this case, y is 2 .
Power Bracer adds 4 EVs to every battle, add that to the 2 for each Metapod/ Kakuna you defeat, you get 6 EVs for every battle fought.
(2 + 4) * 2 = 6 * 2 = 12
Smogon's EV training guide - Pokerus