Jolteon

Volt Absorb
Heals 25% HP when hit by an Electric attack.
Type Tier
Electric OU
Statistics
Min- Min Max Max+
HP
65
- 271 334 -
Atk
65
149 166 229 251
Def
60
140 156 219 240
SpA
110
230 256 319 350
SpD
95
203 226 289 317
Spe
130
266 296 359 394
Name Item Nature

Special Sweeper

Leftovers / Petaya Berry Timid
Moveset IVs
~ Thunderbolt
~ Hidden Power Ice / Hidden Power Grass
~ Substitute
~ Baton Pass
30 Atk / 30 Def
EVs
4 HP / 252 SpA / 252 Spe

Substitute blocks status and can be used to Baton Pass to immunities on switches. For instance, let's say the opponent leads with Milotic. You predict a switch to Blissey, and Substitute, Baton Passing to Gengar. Gengar's Substitute doesn't break, and you get a free Focus Punch even if they have Ice Beam. You can also do this with Skarmory or Tyranitar if they switch to Snorlax / Celebi.

Leftovers will allow Jolteon to use Substitute a few more times, as well as simply letting it live longer, especially in Sand Stream. Petaya Berry is sure to give you something useful, +1 Special Attack, and if they go to something you can't kill, such as Blissey, you can still always pass it along to a mixed sweeper like Blaziken.

The IVs given are for Hidden Power Ice.

Name Item Nature

Agility-passer

Leftovers Timid / Modest
Moveset EVs
~ Thunderbolt
~ Hidden Power Grass / Substitute
~ Agility
~ Baton Pass
4 HP / 252 SpA / 252 Spe

Infinitely better than Ninjask passing if you can predict switches to get the Agility in. Since Skarmory and Suicune can't come in to Roar you away, you are free to Baton Pass to whatever you wish.

EVs on this one can be pretty varied. If you are using Substitute, one idea is to pump Jolteon full of Special Defense so that Jolteon's Substitute doesn't break to the likes of Zapdos Hidden Power. With max (252 EVs, but no nature) Special Defense, Jolteon's Substitute will usually survive even Modest 252 Special Attack Zapdos. Fortunately for Jolteon, many Zapdos are using a lot less than that for EVs (some even use 0 EVs and Modest, with some going even more extreme and using a Calm Zapdos), so Jolteon's Substitute is going to be more than sturdy enough. If you really want to risk it, you can lower Jolteon's Speed quite a bit, because it will generally Agility on the first turn, allowing it to be faster than anything, but don't lower it too much, you still want to be able to use Jolteon as a finisher for some weak Pokémon.

Other Options

You can Tickle / Charm on switches to Snorlax or Celebi to make it easier to kill when you switch Pokémon. Jolteon forces switches like nearly nothing else. Roar allows Jolteon to wall Raikou all day, unless it's the last Pokémon, and can be very annoying with a few layers of Spikes down, thanks to Jolteon's switch forcing ability. Jolteon can Wish pass, and although it isn't as sturdy as Vaporeon or Umbreon, it can switch in on Electrics and Waters to get some free turns. A Jolteon with Thunder Wave can be quite annoying, but Jolteon generally has better things to do than that, and doesn't benefit from the Speed drop.

EVs

Usually you will want at least 374 Speed, the rest going in Special Attack. Maxing out Speed could be a good idea to outspeed non-Jolly Aerodactyl or to tie Jolly Aerodactyl (394). Any leftovers should go in Special Defense or HP.

Opinion

A prime Baton Passer. If you don't need to use Baton Pass, you should probably go with Raikou for your Electric needs. Jolteon's switch forcing ability lets it setup moves or Wish easily.

Counters

Blissey is an obvious counter due to its role as a special wall. Celebi and Venusaur are not bothered by Hidden Power Ice due to their high Special Defense and HP. Porygon2 and Gardevoir can use their Trace abilities to copy Volt Absorb and become immune to Electric attacks. Camerupt and Steelix are the only Ground-types not weak to Hidden Power Grass OR Ice. After a Calm Mind, Raikou's Substitutes will become unbreakable from one of any of Jolteon's attacks. Finally, Swampert can take care of Jolteon that don't have Hidden Power Grass, and Flygon blocks those without Hidden Power Ice. However, note that Jolteon is capable of Baton Passing out of any counter, as it is tied with Aerodactyl as the fastest common Pokémon.