The question said Umbreon, Porygon2 with Eviolite, or Dusclops with Eviolite, so I thought that Eviolite was specified only for the last two Pokemon?
It would depend on the rest of the team (as most things do). If you have Blissey/ Chansey, go for Dusclops to wall Fighting moves that will KO your Cleric. Dusclops has access to Will-O-Wisp, which means it will be able to take physical moves with ease, and can ruin a Scizor set. However, it is susceptible to status moves itself, and doesn't have a reliable method of recovery outside of Rest (it has Pain Split, but it's not reliable). It also forfeits Leftovers recovery in need of Eviolite. It's also really lacking in offensive presence as well, so if you want a Ghost type with a higher offensive stat, go for Cofagrigus.
EVs: 252 HP, 180 Def, 76 Sp. Def
Cofagrigus is another UU Pokemon (I assume this is UU we're talking about), and it can act as an opposite to Chansey, being a physically defensive Pokemon that will be able to take Chansey's weakness. However, it has no reliable method of recovery outside of Rest (again, it only has Pain Split has its other option), but it does have the very useful Haze, so even if it did lack offensive presence, it will be able to reset calm mind/ bulk up stallers.
EVs: 252 HP, 252 Def, 4 Sp. Def
Pain Split/ Knock Off
If you have, say, Mew or Slowbro, then go for Umbreon. Umbreon has reliable methods of recovery in Wish & Moonlight, in addition to Heal Bell for team support. Umbreon is really lacking in offensive power, with its most viable move being Foul Play, which means that it lets special attackers like Togekiss to set up. The up side is that Umbreon has Synchronise, so paraflinchers like Jirachi and Togekiss will get paralysed themselves.
EVs: 252 Def, 252 Sp. Def, 4 HP
For Porygon2 (aka Duck Tank), the best ability for it to successfully wall your enemy will be Trace. Porgon2 has good offensive power, even though it has a shallow move pool, so it will be able to fight back while taking some hits. If it gains a good ability with Trace (like Sap Sipper or any other type-negating ability), it can cause your opponent to switch Pokemon which will buy you a free move. The downside is that it's slow, has no resistances (bar its Ghost immunity) and is weak to fighting like Blissey. While it forfeits leftovers, it has Recover to replenish health.
Nature: Bold/ Calm
EVs: 252 HP, 180 Def/ Sp. Def, 76 Sp. Atk (you can make Porygon2 defensive or specially defensive, depending on what your team needs)