It's 6th gen now so I am going to review all the eeveelutions.
Vaporeon dropped from OU and now shines in UU. Most Vaporeons you will meet are clerics or walls/stallers with moves like Wish, Heal Bell, Toxic, Scald, and Protect.
Jolteon also dropped from OU and is good in UU. All Jolteons you will meet are fast special sweepers with moves like Thunderbolt, Signal Beam, Shadow Ball, Hidden Power, and Substitute.
Flareon has had some buffs(mainly Flare Blots and fairy resist) but is still the worst eeveelution by far. Flareon can be offensive or defensive. Defensive Flareon packs Wish, Protect, Heal Bell, and Lava Plume while offensive Flareon usually has the abillity Guts and packs moves like Flare Blitz, Flame Charge, Facade, Bite, Iron Tail, Superpower, and Protect.
Espeon is still OU and OK. Espeon is always a fast special sweeper with moves like Psychic,, Psyshock, Dazzling Gleam, Shadow Ball, Calm Mind and Yawn. Its main niche is Magic Bounce.
Umbreon is still UU and OK. Umbreon is always an annoying stalling with moves like Toxic, Moonlight, Curse, Payback, and more.
Leafeon is superior to Flareon in that it works but has a very small movepool, which is what holds it back, along with its typing. Most Leafeons are Sword's Dancers with Substitute, Sword's Dance, Leaf Blade, then filler or coverage.
Glaceon is right next to Flareon in terms or performance and is outclassed by Jolteon and Espeon. Just give him 4 attacks and Choice Specs or Life Orb.
Sylveon, the new one, has replaced Vaporeon in OU and here is why: Vaporeon's strongest strategy then was Hydration. It immediately heals Vaporeon's status conditions in rain. A lot of Vaporeons would carry Rest and be on the same team as Politoed but now that the weather has been nerfed, Vaporeon lost its greatest assets. Sylveon came to OU because fairy is excellent typing and he is easier to EV/Super Train. If you wanted a mixed wall, max out HP and Def with a Bold nature. If you want a special wall, max out HP and SDef with a Calm nature. Sylveon is just a better cleric than Vaporeon this gen. Most Sylveons are clerics with moves like Wish, Heal Bell, Toxic, Protect, Moonblast, and Hyper Voice. I mentioned Hyper Voice because it breaks through substitutes and gets boosted by Pixilate.
Now, let us rank them:
Special Sweeper: Espeon/Jolteon
Physical Sweeper: Leafeon
Runner up(s) to Special Sweeper: Glacieon
Runner up(s) to Physical Sweeper: Flareon
Runner up(s) to Cleric: Vaporeon
Umbreon has no competion.
I hope I helped :)