Gen 8 removes Signal Beam and Hidden Power, leaving Jolteon with only a few options.
With a great 130 speed stat, Jolteon is currently the 10th fastest Pokemon, but his movepool has still got a ways to go.
Anyway, on a weather reliant team, a Jolteon could have this set:
Jolteon (M) @ King's Rock
Ability: Volt Absorb
EVs: 20 HP / 233 SpA / 4 SpD / 252 Spe
- Thunder/Thunderbolt (100% accuracy in the rain, STAB/main STAB)
- Weather Ball/Substitute (Changes type according to weather to fit coverage/protection)
- Shadow Ball (Coverage)
- Volt Switch (To switch out)
Weather Ball is a nice variable to have, because it can give you some nice coverage if used correctly. It also get 100 base power in weather. For example, in a rain team, Jolteon will be able to cover common ground and rock types, such as Excadrill, Dugtrio and Tyranitar. If you don’t go for it, Substitute works as well. Just make sure you give him Leftovers so he can recover a bit.
Thunder is best if you decide to go on a rain team (in which case Weather Ball will be Water type), if not, Thunderbolt is the default option for STAB.
Shadow Ball is just default coverage against some common Ghost types like Gengar.
Volt Switch is for when you need to escape.
A King’s Rock is nice because it’ll give all his moves a 30% chance to flinch, and since Jolteon outspeeds so many Pokemon, the flinch chance could (if you decide to go into Doubles) give Jolteon’s teammate a chance to go in for the kill. The key here is to make sure Jolteon doesn’t die, because Jolteon’s stats don’t exactly scream Tank. The alternative is to give him Leftovers, so he can recover at least a little bit.
I hope this helps someone (even if this is a messy, long post.)