I think these are the three fastest methods (not so much the Pokérus method though, because you have to find a Pokémon with Pokérus).
Method 1:
The Power Items, are held items you can give your Pokémon to increase
the EVs they gain from battle. Note that these give a specific stat
EVs regardless of the Pokémon defeated. So if you defeat a Zubat with
a Pokémon holding a Power Weight, it'll get +1 Speed EV and +8 HP EVs,
too.
Source
The items above are located at Battle Royal Dome, Royal Avenue for 16 Battle Points (BP) each.
Method 2:
Use vitamins. Vitamins will give your Pokémon 10 EV points in a
particular stat. You can give your Pokémon up to 10 vitamins, meaning
that this is a fast way to get 100 EV points (out of the limit of 510
EV points). Vitamins cost $9,800 each.
In Sun/Moon/Ultra Sun/Ultra Moon, you can buy Vitamins in the Pokemon Center on Mount Hokulani, or the Battle Royale Dome for 2 Battle Points (BP) each, or at a 3-star Pharmacy at the Festival Plaza.
Method 3:
Try to contract Pokérus. Pokérus is a Pokémon virus. Though rare,
Pokérus is a status affliction that cannot be cured via Pokémon
Center. Although it is possible to battle a Pokémon with Pokérus
(which can only be received by said method by battling with a Pokémon
and not by running), you will most likely have better luck receiving
one over trade. Much like a regular virus, once one of your Pokémon
catches the virus, it can spread to your other Pokémon. This virus
will double the EVs you get from battles and works with the other
items which increase EVs gained. However, your Pokémon will only have
it for a certain period of time and then they will heal, usually
around a day after contracting Pokérus. Find out if your Pokémon has
Pokérus by checking their status effects. You'll also be told by the
nurse at a Pokémon Center if your Pokémon has Pokérus. Keep in mind
that this virus is very rare. You may never encounter it.
Source
Methods 2 and 3 have the same source.
Method 4: horde battles (gen 6 only) or SOS battles (gen 7 and higher).
Method 5: Use the Shiny Ditto Glitch.
