Surprised no one has posted these:
Celebi @ leftovers
Ability: Nature Cure
EVs: 172 HP, 152 Sp. Atk, 184 Speed
- Leaf Storm
- HP Ice
Heatran @ Leftovers
Ability: Flash Fire
EVs: 240 HP, 252 Sp. Def, 16 Speed
- Lava Plume
- Stealth Rock/ Roar
Celebi and Heatran cover each other's weaknesses, Celebi being able to take Ground, Water and Fighting hits, and Heatran being able to wall every type Celebi is weak to.
Heatran doesn't really need Sp. Atk investment to be able to deal some good damage, so it can be made defensive while having some offensive presence.
UU: Cofagrigus/ Porygon2
Cofagrigus @ Leftovers
EVs: 252 HP, 252 Sp. Def, 4 Def
- Shadow Ball
- Pain Split
Porygon2 @ Eviolite
EVs: 252 HP, 252 Sp. Atk, 4 Def
Nature: Modest/ Quiet
- Ice Beam
- HP Fire/ Fighting
Cofagrigus actually works quite well as a sp. def wall, since it's sp. def isn't that shabby, and it already has Will-O-Wisp to deal with physical attackers, in addition to Mummy negating things like Technician, Guts, Sheer Force or Skill Link. It also negates set up Pokemon/ Baton Passers with Haze, which is a really useful move.
Cofagrigus will provide the Fighting immunity, Porygon2 will provide the Ghost immunity and the Dark neutrality. Por2 has enough bulk to wall a few neutral moves as well as deal some pretty substantial damage with Analytic.
Blissey/ Chansey will also work in place of Porygon2, but that'd have to OU and it would mean that neither Pokemon would be able to deal much damage (unless you Toxic stall).