Umbreon

ToxiStaller

Umbreon @ Leftovers
Bold / Impish
252 HP / 160 DEF / 96 Sp. DEF

-Mean Look
-Toxic
-Taunt / Protect
-Wish

This is a slow killer, but very effective at it. Basically, Mean Look something and Toxic them to death. Umbreon's pretty bulky and Wish makes sure that he'll stay alive for quite a while. Taunt is there to stop the trapped Pokemon healing or setting up, but Protect is a decent second option to improve Wish recovery.

The EVs are very variable here. Maximising HP is generally advised, though how much you want to invest in each defense is up to you. This set is more orientated towards blocking physical hits, due to its Fighting weakness, but you can invest however you want really. The nature can also be any +DEF or +Sp. DEF nature that suits you.

Counters
Steel and Poison types completely wall this set as they're unaffected by Toxic. Heracross, and anything else with Guts, can really ruin Umbreon here as they get a free boost and can strike back hard while he's powerless to do anything.

Taunters also walk all over this set as they can set up on Umbreon and then sweep.

Unless you get a Taunt in before the opponent (you'll probably use Protect for Wish anyway) then the opponent can Taunt you and ruin this set.

EDIT: Just noticed it already said in the Counters section