First things first, as Fizz pointed out, Eevee has low special attack and its special defense is its highest stat, meaning that really, with the information we know, it could effectively evolve into any of the 8 eeveelutions. I’ll make this answer for Gen 8.
Ok, so now that I’ve gotten that out of the way, it comes down to the IVs and Nature of your Eevee. I assume this is in game though, and I’ll point out that Umbreon isn’t the best option in game. A lack of offensive stats makes it outclassed by every other eeveelution in game, due to them all having a high offensive stat (for special attack there is Sylveon, Flareon, Vaporeon, Jolteon, Glaceon and Espeon. For Attack there is Leafeon and Flareon).
Anyway, as far as IVs and Natures go, if you have, let’s say, 24 Special Attack IVs, 30 Attack IVs, 28 Speed IVs, and Jolly Nature, Flareon/Leafeon would be the best option (by the way these are all random). Here are my suggested natures for each eeveelution:
Note: Hardy/Docile/Bashful/Quirky/Serious nature’s will depend on IVs so I’m not counting them, since they are all neutral natures.
Flareon- Special Attacker: Timid/Modest. Physical Attacker: Jolly/Adamant.
Jolteon- Timid/Modest.
Vaporeon- Modest/Bold/Calm.
Espeon- Modest/Timid.
Umbreon- I don’t recommend it, but Bold/Calm/Careful/Impish could work depending on the set.
Leafeon- Adamant/Jolly.
Glaceon- Modest/Timid/Calm/Bold, depending on the set.
Sylveon- Modest/Bold/Calm.
Anyway, these are just some recommended nature’s for them. Basically, it boils down to the IVs, but choose what you’d like. Again, just some suggestions.
Hope this helps! :)
