I’m going to start off by saying I’m going to use common Pokémon that are easy to SOS chain that I’ve seen in guides.
HP- Caterpie. With a 35% encounter rate in the grass patches nearest to Iki Town on Route 1, it shouldn’t be that hard to find. Each Caterpie you beat grants you 1 HP EV. Caterpie are also really easy to beat. If you want to get more EVs than that, chaining Chansey in Poni Plains gives you 2 HP EVs per Chansey and 3 per Blissey.
Attack- Yungoos. In the same patches of grass as Caterpie most commonly spawns, there will be a 35% encounter rate for Yungoos. Each Yungoos gives out 1 Attack EV when you beat it. Can only be encountered at daytime.
Defense- I’d say Bonsly. If you break the rocks on Route 1, enter the grass and there is a 30% encounter rate for Bonsly. It grants, again, 1 Defense EV. It can also call Sudowoodo as its ally, which gives 2 Defense EVs when you beat it.
Special Attack- This one is harder. I’d suggest Zorua though. At the Trainer’s School, Zorua has a 30% encounter rate. Beating a Zorua gives 1 Special Attack EV. It can only call itself when SOS chaining though.
Special Defense- I’d go with Drowzee. On Route 2 it can be encountered via ambush encounters (30% at night, 5% in the day). It can also be encountered with a 10% in the day and 20% at night rate, except in the most north patch of grass. It grants 1 Special Defense EV.
Speed- The easiest way to get speed EVs is by chaining non other than Zubat. It can be encountered at a lot of cave places, but I’ll use Seaward Cave for the encounter rate. While walking in the cave, it has a 70% encounter rate. While surfing, that number is bumped up to 80%. Each Zubat you knock out grants 1 Speed EV.
Hope this helps! :)
