One of the best Eeveelutions out there, Umbreon's main halmark to fame from the moment it was released back in generation 2 was its incredible defenses. No more high HP but low defenses or vice versa, we are talking 95, 110, 130 defenses complimented with pure Dark typing which makes Umbreon hard if not impossible to one shot even in the upper tiers. And fortunately Umbreon is also blessed with an excellent supportive movepool to get around its low offensive stats even in spite of losing the Toxic TM in between generations, making it an all around excellent choice to fit onto your competitive team.
Be Sturdy and Play Dirty
Ability: Synchronize
Item: Leftovers
Nature: Impish (+Def -Sp.Att)
EVs: (HP 252) (Def 156) (Sp. Def 100)
Moves:
Foul Play
Wish
Protect
Confuse Ray/Yawn/Body Slam/Toxic
Set Details:
Umbreon being a defensive and pure Dark type means that Foul Play was practically made for it. No need to invest anything in your own offenses when you can just hijack your opponent's stats using this unique and powerful STAB. Any good defensive set also needs a form of recovery, and Wish is your best option. Not only can it heal half your HP, but you also have the option to switch out and let your teammate benefit from the meaty recovery instead. Protect buys you a turn for Wish to activate, nets you extra Leftovers recovery, and scouts the opposition. Your last choice is a toss up depending on what you want. Toxic is obviously ideal since it pairs well with every other move and accumulates damage as you stall for time, but if you can't transfer it over you can annoy your opponents with Confuse Ray, offer sleep support with Yawn which also pairs well with Protect, or you can slap on Body Slam if only for the 30% paralysis support.
Cursed Fox
Ability: Synchronize
Item: Leftovers
Nature: Careful (+Sp. Def. -Sp.Att)
EVs: (HP 252) (Def 4) (Sp. Def 252)
Moves:
Curse
Crunch/Payback
Protect/Body Slam
Wish/Moonlight
Set Details:
A set up tank set shores up your offenses while making you harder to KO overall. Boost your attack and defense with Curse at the cost of speed, and take out your opponent with either Crunch which has more consistent power and PP or Payback which benefits of having more power once you go second but has lower PP. Protect stalls, scouts, and buys extra Leftovers recovery, but Body Slam offers neutral coverage against Fairy, Fighting, and other Dark types with added paralysis support that your team can exploit. Finally we have your mandatory recovery move with Wish that pairs well with Protect and offers recovery to your other teammates, but Moonlight requires no charge time and has the potential to recover even more health with sun support, but it still has lower PP than Wish and can be nerfed if the weather is rain, hail, or sand.
There is still some debate over whether Espeon or Umbreon is the best Eeveelution, but you can always pop both on your team in the meantime until the debate is settled ~_^