Based on a Lavos's skeleton and fleshed out by me.
As a pure Electric type, Jolteon faces heavy competition from Raikou due to its better bulk and power, which means that Jolteon is outclassed by it as a special wall and as a phazer. Furthermore, the popularity of other Electric types such as Zapdos makes Pokemon like Steelix and once again Raikou popular, which hinders Jolteon even more.
However, there's still a reason why Jolteon is OU : Baton Pass. Alongside boosting moves such as Agility or Growth, a sky-high Speed stat and an Electric STAB which allows it to pressure common phazers like Skarmory and Suciune, Jolteon makes a very effective user of this move.
Set 1: Baton Pass
Move 1: Baton Pass
Move 2: Agility / Growth
Move 3: HP Water / HP Ice
Move 4: Thunder / Thunderbolt
This set exploits Jolteon's main advantages over Raikou, with Baton Pass being the selling point. Indeed, Jolteon itself doesn't benefit from Agility due to its natural Speed, it's meant to be passed to a slower Pokemon that could attempt a sweep. However, Jolteon becomes more individually threatening with Growth, because of its Speed and of the damage output by boosted Thunder. The nature of the Hidden Power depends of your team, but since Steelix is one of Jolteon's biggest problems, Hidden Water is often the preferred option as it allows you to 3HKO this annoyance, as well as doing a plenty of damage to Rhydon and Golem which are even more threatening offensively. Hidden Power Ice, on the other hand, gives you a better overall coverage and dents dangerous Pokemon like Exeggutor. In the same vein, the choice between Thunder and Thunderbolt is based on personal preference. Thunder hits much harder, especially when boosted by Growth, and can land paralysis on opposing Pokemon like Raikou, which can be very rewarding depending on your team, whereas Thunderbolt is a weaker but safer option.
Jolteon's access to multiple boosting moves can be worrying at first for your opponent, but keep in mind that once you've reavealed it, he can get a pretty clear idea of the remaining members of your team if you haven't showed them already. Indeed, Jolteon is likely paired with slow and powerful Pokemon if using Agility, or other special attackers if using Growth. In the former case, common partners include Machamp, Belly Drum Snorlax and especially Marowak, with which it forms a well-known core called "JoltWak". Growth Jolteon's teamates, however, will probably not sweep an entire team at +1, but Pokemon like Exeggutor, Gengar and Jynx will be potentially able to overpower their habitual answers, especially if they manage to put them asleep. Nidoking deserves a special mention for its ability to immediately threaten Raikou, the arguably best special wall overall, while still beneficing from Growth since boosted Ice Beam and Thunder are much more punishing for Zapdos and Suicune, respectively. Finally, Jolteon is extremely common on dedicated Baton Pass teams, alongside Smeargle and to a lesser extent Scizor and Umbreon.
Jolteon can try to pass Substitutes, but it’s so frail that they will almost always break with any attacking move. Although it's outclassed as a defensive oriented phazer, Jolteon can still make a good use of Roar by lowering its Speed below 328 in order to outslow Raikou and preventing it from phazing you first. This move can also be a pain for other answers like Snorlax and Steelix if used in tandem with Spikes. Reflect and Charm softens powerful physical attackers like Snorlax and Marowak which are tempted to come on Jolteon.
[Checks and Counters]
Preventing Jolteon to pass any boost is the best way to check it. Roar Raikou is arguably Jolteon's worst nightmare, since it cannot threaten it offensively and will have to dry pass as long as the opposing Raikou is awake. Steelix is another phazer which deals heavy damage with STAB Earthquake if Jolteon try to stay in, but it takes a significant amount from Hidden Power Water. Tyranitar is bulky enough to withstand three Thunder most of the time, enabling it to roar Jolteon out at least once, but just like Steelix, it will have to check other Pokemon and thus can be worn down pretty quickly. Outside of phazers, Pokemon that can take Jolteon's hits and threaten and the recipients alike are very annoying as well. Snorlax, Exeggutor, Nidoking and Marowak are all common offensive Pokemon that can either throw powerful hits, spread sleep and paralysis, explode or boosting themselves in front of Jolteon. Finally, most special walls can disable Jolteon too since it's doesn't hit hard enough to keep them at bay. Blissey can threaten any recipient with Sing and to a lesser extent Toxic, Light Screen variants are especially painful against teams based around Growth pass. Umbreon ensures that physical oriented recipients won't pose much of a threat with Charm being unavoidable since Baton Pass also transfers negative stat changes, but it has to watch out for the potential paralysis. In the same vein, Meganium and Venusaur, although uncommons, can cripple anything coming freely with Leech Seed and with either Screens or Sleep Powder, respectively.