Here's what I think works best (in Sun/Moon):
[email protected] Plate/Wide Lens
Ability: Volt Absorb
Nature: Timid (+Spd, -Atk)
EVS: 252 SpA, 252 SpD, 4 HP
-Volt Switch/Rain Dance
Why I think this:
Zap Plate gives a boost to your 2 or 3 Electric type attacks, giving them even more power. Wide Lens helps with Thunder.
Thunderbolt is a great Electric type attack, 90 power and 100 accuracy. You can choose this, or you can choose Pin Missile if you want to deal with Grass types. Hidden Power Ice works well here too, except I don't find Hidden Power that strong of a move. No STAB, no item boost. Bad move to use.
Volt Switch is if you want to switch out due to enemy Ground types and still be able to attack. On my Jolteon, I find it does intense damage. Only works in Double Battles or Battle Royals. Or you can choose Rain Dance to allow Thunder to hit all the time (provided no accuracy lowered).
Shadow Ball is so that if you choose Thunderbolt, you won't have four Electric type attacks. I find it necessary though. I highly doubt Pin Missile works here. Low Attack to begin with, plus its nature.
Thunder goes without saying. It does EXTREME Electric-type damage. If you want to, you can replace
In Double Battles, I find that Jolteon goes well with Tapu Koko. Koko's ability, Electric Surge, increases the power of already extremly powerful attacks. As a bonus, Tapu Koko knows Discharge, which can heal your Jolteon with its Volt Absorb.
I find this Jolteon better for PvP.
Hope I helped!