Best Eeveelutions

Dark is my second favorite type. I simply love Umbreon and Espeon.
Umbreon is often ranked as either the strongest or second strongest Eeveelution because of how tanky it is. This Pokémon would be incredibly challenging to defeat.
And if it needs to go on the offensive, just use Foul Play. Foul Play calculates damage based on the opponent's attack rather than your own. Synchronize and Inner Focus are both pretty good abilities.
I like Dark-type Pokémon, and this one is just amazing - an unlikely tank Pokémon that can have a pretty good moveset. I think Umbreon, as well as the non-Gen 1 Eeveelutions, should have been in Pokémon Let's Go Pikachu and Eevee. The game was ruined because of the plain Gen 1 approach and no postgame at all.
With all the games I've played in the past, I have always tried to get Umbreon on my team because it's my favorite Pokémon. In other words, Umbreon is just plain awesome and is very good in battle despite its bad attack and special attack. I just wish they didn't make its max CP so low in Pokémon Go compared to other Eeveelutions.

Espeon is definitely either my favorite or second favorite. Umbreon would take the other spot.
Anyway, Espeon is fast, graceful, and powerful. You just have to use it right. It's best suited for facing fellow Special Pokémon, and it has simply stunning abilities. Dazzling Gleam is excellent for Dark-type coverage.
In reply to one of the comments, Umbreon isn't overrated, and neither is Sylveon. Sylveon is pretty good, but not because of ribbons and bows. Sylveon's lack of speed barely affects it - it is still either the most or second most powerful Eeveelution.

Sylveon definitely deserves third place. It's fairly cute, with an excellent moveset and a really overpowered hidden ability: Pixilate.
Basically, Pixilate turns any Normal-type move into Fairy-type with a 20% damage bonus. Consider Hyper Voice: 90 damage plus 20% becomes 108. Then, STAB will turn that into 162. And Hyper Voice can hit multiple opponents.
With the right combination, Sylveon is practically unstoppable.
Sylveon is just adorable. I actually have a shiny Sylveon, and it's literally epic. Umbreon is way too overrated, in my opinion. Sylveon is a great special attacker while also being a tanker on special defense. And its shiny is blue! Blue and pink just go so well together!
Besides, if you ask me, Sylveon shows off the blue on a shiny better than an Umbreon. This little dragon killer is a sweeper as well. My shiny Sylveon could even withstand a Sludge Bomb! Spamming Calm Mind works for me to build up power for Draining Kiss.

I totally agree with Vaporeon's fourth-place ranking. It's strong and cute, though not the best.
Vaporeon can basically turn invisible in water and has a great moveset. It's fairly tanky and has high Special Attack. Moves like Muddy Water and Scald make it a strong contender.
In my opinion, Vaporeon is the third most powerful Eeveelution, after Sylveon and Umbreon. Plus, its abilities, Water Absorb and Hydration, are great if you're facing a fellow Water type. AND it can learn Ice Beam without gaining the weaknesses of Ice types.

Now, Glaceon. I completely agree with its placement. The main reason is because its design is really good.
Unfortunately, the Ice type has been considerably weakened by Pokémon over time. It is either the weakest or second weakest Pokémon type. However, Glaceon has a decent stat spread - it's really tanky, which helps it survive, and it has great special attack. Its abilities are solid, too. Just summon some hail, and Glaceon becomes significantly stronger.
With moves like Hail, Ice Beam, and Toxic, it can be really effective. That said, its speed is a bit of a weakness. Glaceon is often regarded as the weakest or second weakest Eeveelution, but that's not its fault. It's the fault of having the weakest/second weakest type and a stat spread that could use improvement.

For me, I think JOLTEON should be in seventh place. It has an original design, I guess, and it is REALLY fast.
However, Jolteon would not be able to survive a good physical attack due to its stat spread. It basically just goes first, hits, and then possibly faints on the next turn. I wouldn't say it's ugly, but I do think it has less appealing aesthetics compared to other Eeveelutions.
It does have some pretty good abilities, but overall, it's not a particularly strong Eeveelution. I'd say it should switch places with Leafeon.

Leafeon is better than Jolteon. Apologies to Jolteon, but I believe the two should switch places. Leafeon has an excellent design that deserves sixth place.
Jolteon is fast, but it's more of a one-hit, one-chance, then faint kind of Pokémon. Leafeon only needs to use Sunny Day, which increases its speed and makes it immune to burns. After that, it can basically destroy opponents with Solar Blade.
Lastly, Leafeon has pretty good stats. It can deal damage and survive hits, plus it's fairly fast. LEAFEON DOES NOT DESERVE SEVENTH PLACE.

I do think Flareon should be eighth. It's fluffy but looks more like an orange Eevee than an original Fire type.
Flareon could have been really effective if Pokémon had given it decent physical moves. Flare Blitz has recoil, which is such a waste of its high Attack stat. They could've made its Speed higher and switched things around so Flareon would be fast with effective Special Attack moves.
I would rank the Eeveelutions as:
1. Umbreon/Espeon
2. Umbreon/Espeon
3. Sylveon
4. Vaporeon
5. Glaceon
6. Leafeon
7. Jolteon
8. Flareon