Flareon @ Toxic Orb
Guts
4 HP/252+ Atk/244 SpD/8 Spe (Speed creeping on any Flareon switches that want to speed creep on uninvested opponents)
-Flare Blitz
-Facade
-Bite
-Flame Charge/Quick Attack
Flare Blitz is main stab, but against things like Espeon, Facade kills as well
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52+ Atk Guts Flareon Flare Blitz vs. 4 HP / 0 Def Espeon: 487-574 (179 - 211%) -- guaranteed OHKO
252+ Atk Guts Flareon Facade (140 BP) vs. 4 HP / 0 Def Espeon: 379-447 (139.3 - 164.3%) -- guaranteed OHKO
However, Umbreon almost OHKOs with Foul Play, so unless you catch it on the switch you can't do much.
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252+ Atk Guts Flareon Flare Blitz vs. 252 HP / 4 Def Umbreon: 297-349 (75.3 - 88.5%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Guts Flareon Quick Attack vs. 252 HP / 4 Def Umbreon: 67-79 (17 - 20%) -- possible 7HKO after Leftovers recovery
This option gives you something like a 60% chance to KO on the switch by spamming Flare Blitz and then finishing opponents with Quick Attack as priority.
And against Jolteon:
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252+ SpA Choice Specs Jolteon Thunderbolt vs. 4 HP / 244 SpD Flareon: 160-190 (58.8 - 69.8%) -- guaranteed 2HKO after toxic damage
252+ Atk Guts Flareon Facade (140 BP) vs. 4 HP / 0 Def Jolteon: 379-447 (139.3 - 164.3%) -- guaranteed OHKO
Also, against a Flareon:
Spoiler, click to toggle visibilty
252+ Atk Guts Flareon Facade (140 BP) vs. 4 HP / 0 Def Flareon: 379-447 (139.3 - 164.3%) -- guaranteed OHKO
In the case of this, you may want to run 252 Spe just to make sure you can hope for a speed tie on other Flareon and OHKO with Facade.
And since it appears that the most used variant of Flareon is Jolly 252 Spe, you may run into that quite a lot.
And as seen elsewhere, Facade will be your best option against Vaporeon, scoring a 2HKO against fully invested sets. And by speed creeping, your guaranteed a safe battle on a switch into it if you spam Facade. Considering most threats aren't taking much damage from Flare Blitz, it may be safer to spam Facade as your main offensive option, especially on teams with Vaporeon. You also outspeed uninvested Umbreon and 2HKO with Facade. In fact, your biggest threat while spamming Facade is actually Leafeon, who almost always survives after the switch, and deals 50% with Knock Off, and can either try to protect stall go for the 2HKO with it.
So, after I've written this well thought out analysis paper, you can deduce that your main issues using Flareon are going to be Leafeon, Umbreon with speed investment, or other Flareon running speed investment. Also, if you run mixed defenses, even a bastard set with 124 in SpD, you can have a chance of surviving two fully invested Psychics from Espeon and OHKO with anything not named Quick Attack. However, should you decide to run Flame Charge, you remove most speed threats after three uses and after that speed issues aren't an issue on potential switches.
So, for some reason I wrote you a research paper on Flareon. Enjoi