You really need Timid or modest and Timid is perfered Jolteon is mainly used for it electric immunity, it speed, and it s.atk. at top speed I think it ties with Crobat and Mewtwo which means unless the opponent is boosted or using Choice scarf Jolteon well most likely outrun them. Timid is the best nature as it allows Jolteon to reach top speed.
My Jolteon has Hidden power(ice 68-69 power so not bad) Thunderbolt, Wish(egg move may remove it), and Baton pass(only Eevee learns it but may remove for Volt switch or Signal beam.) It also has max hp, speed, and s.atk IVs and its EVs are 6 hp 252 s.atk and 252 speed so it reachs its tops speed its top neutral special attack and has decent hp.
If you going to use a Jolteon try to get it close to that although max hp IVs are not required(just got very lucky thanks to my male vulpix with max IVs in everything but atk and def and def has 30 IVs

) and hidden power is a pain to get unless you lucky.
Jolteon
Nature: Timid
Ability: Volt absorb
Item: Life orb/Leftovers/Expert belt
Move:
-Volt switch(Just a good move and works well on Volt turn team)
-Thunderbolt/thunder(main stab attacks. thunder in rain)
-Signal beam/substitute/Rain dance(Signal beam for coverage, Substitute for when you force a opponent out to try to 2 hit ko a counter, and Rain dance for a 2nd way to start rain on a rain team)
-Shadow ball/Substitute/Rain dance/Hidden power(Shadow ball for coverage, Hidden power Ice at 66-70 gives you the best coverage but difficult to get)