RBY - Although UU, Jolteon is perfectly viable in OU as a mixed attacker. Its very high Speed (it's the fastest thing in RBY along with Aerodactyl), and as such very comparatively high critcial hit rate, fare it well. Jolteon has fairly decent coverage as well - Double Kick for Chansey (and a very weakened Golem/Rhydon), and Pin Missile for the various Psychics (notably Exeggutor) and it also does quad damage to Victreebel and Venusaur in RBY. However, Jolteon is quite easily walled -Golem and Rhydon will need wearing down if you want to clean up late-game with it, Double Kick will need to score a critical hit on Chansey to inflict any significant damage that cannot be simply Recovered off, and often when using Jolteon in RBY, prediction must be sharp, because many of Jolteon's switch-ins are an immediate threat, either inflicting the paralysis Jolteon loathes, or firing off a Super Effective Earthquake. Overall - Jolteon is a decent enough late-game cleaner, although not as much so as the exalted Tauros or the crit-monster Persian.
GSC - Still learning this gen (through reading Borat's stuff and pretty much stalking M Dragon on PO whenever he plays GSC), but I know that Jolteon only ever sees use in GSC as an Agility passer in the JoltWak combo.
ADV - Jolteon's main role is still as a Baton Passer, although this time it's Baton Passing something else (Substitutes) or indeed nothing at all (dry passing). Jolteon is a great scout in ADV, and also one of the very best leads, forcing many other common leads right out of the fray and quickly Baton Passing a Sub to a powerful mixed or physical sweeper such as Blaziken or Gyarados, or simply "dry passing" on a predicted switch. But that's not all! Jolteon can also completely wall Raikou with Roar above Substitute on its set, and pass Wishes effectively around the team. Jolteon loves Spikes support, due to its ability to force switches (and phaze on predicted switches) and also the fact that nearly all of its counters are grounded. Overall, Jolteon is a top-notch scout with decent utility, and can also function as a late-game cleaner of sorts, but it is hurt by all forms of passive damage,and is comparatively frail. Jolteon is low OU in ADV, but it should get more use for its merit.
DPP - Still learning this gen (my first team is an incredibly difficult-to-play with Hail Stall team where the only Ice-type is Abomasnow). After roughly 100 ladder games, I've seen about 5 Jolteons (mostly Specs and 1 Sub-passer) so I don't have enough knowledge to comment about this one. I presume that those two sets are what Jolteon does well in DPP though, along with maybe a Charge Beam set?
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