Don't use this:
Jolteon @ Choice Specs / Life Orb
Ability: Volt Absorb
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Thunderbolt
- Volt Switch
- Shadow Ball
- Hidden Power Ice / Signal Beam
Why it's bad: With many Pokemon introduced, revamped, or brought back to OU, most with far better stats, abilities, and movepools than seen before, Jolteon's main niche, a fast 130 base speed, is no longer as exclusive as it was before. Furthermore, the lack of permanent rain has severely undermined Jolteon's power by preventing it from spamming Thunder. With a severely lacking base 110 Special Attack, extremely thin defenses, and one of the worst STABs to be locked into, Jolteon can no longer find a place in OU, and using it is often only a liability to the rest of your team.
Instead, use this:
Thundurus @ Life Orb
Ability: Prankster
EVs: 4 HP / 252 SpA / 252 Spe
Timid / Naive Nature
- Thunder Wave
- Thunderbolt
- Focus Blast / U-turn
- Hidden Power Ice / U-turn
Why it's better: Thundurus carries not only a substantially higher Special Attack stat, but an arguably superior typing that gives it far better of a matchup against the Ground-types that typically wall Jolteon. Thanks to Prankster, Thundurus can more than make up for its slightly lower speed in comparison to Jolteon by being able to utilize a priority Thunder Wave, allowing it to outspeed even the fastest of attackers. With just two attacks Thundurus can attain near-perfect coverage, and still have room for either another coverage move or the momentum-grabbing U-turn. With a powerful Focus Blast, Thundurus is not plagued by common special walls such as Heatran and Tyranitar. With a jaw-dropping array of options ranging from priority Taunt to physically based Defiant + Bulk Up sets, Thundurus is also many times more flexible than Jolteon and can easily fill a number of roles.
Conclusion:
If you're looking for a fast, powerful, and adaptable Electric-type attacker, look no more to Jolteon and instead embrace Thundurus. With better stats, typing, and a movepool that Jolteon would die for, Thundurus fulfills all of the roles that Jolteon does and much, much more. Overall, Thundurus brings to the battle field a much more threatening presence that can affect the whole tier.