Continuing from last night when I didn't have time to finish (and I just closed my webpage so I have to start all over again [RAGE])
Physically Defensive: Giratina-A
Giratina @ Leftovers
EVs: 248 HP / 252 Def / 8 SDef
- Sleep Talk
- Roar/Dragon Tail
This Pokemon can tank any physical attack at full HP, and survive it. Capable of ruining Pokemon with a combo of Wow and Dragon Tail it causes a huge number of ragequits. Sleep Talk and Rest provide it with some great recovery and allow it to operate all the time.
252 Atk Choice Band Teravolt Zekrom Outrage vs. 252 HP / 252+ Def Giratina: 416-492 (82.5 - 97.6%) -- guaranteed 2HKO after Leftovers recovery
Arceus @ Leftovers
EVs: 248 HP / 252 SDef / 8 Def
- Cosmic Power/Roar
This Arceus is similar to the Specially Defensive Giratina-A spread, allowing it to burn physical attacks and stall everyone. Normal Arceus is chosen so that it can carry Leftovers, which are good in case of residual damage. Recover is your HP recovery, while Cosmic Power can be used to stall, or Roar to phaze. Return is in case of Taunt and can deal some nice damage.
Specially Defensive: Blissey
Blissey is capable of taking any special attack with it's base 135 Special Defense and its 255 base HP stat. It makes great team support with moves like Wish and Aromatherapy.
Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 252 SDef / 4 Def
- Thunderwave / Toxic
- Seismic Toss
Seismic Toss is preferable over other moves as it will always do a set amount of damage. Toxic is good for 1v1 situations, although Thunderwave is preferable for team support. Aromatherapy and Wish allow fro more team support curing burns and toxic and healing allies.
Physically Defensive: Skarmory
Skarmory can tank more moves than the other two options, Ferrothorn and Forretress, and also has a reliable recovery move. It also forms a SkarmBliss core which was a highly popular defensive core in Gen 3.
Skarmory @ Leftovers
EVs: 248 HP / 232 Def / 28 Spe
- Brave Bird
- Defog / Stealth Rock
- Taunt / Whirlwind
Speed EVs allow it to speedcreep while also maintaining a strong Defense. Roost is reliable recovery, Brave Bird is in case of Taunt, and also deals some decent damage. Defog or Stealth rock because both is just silly (as Defog will remove all hazards on both sides). Taunt and Whirlwind are both options, really depends on personal preference.
252+ Atk Life Orb Iron Fist Conkeldurr Thunder Punch vs. 252 HP / 232+ Def Skarmory: 172-203 (51.4 - 60.7%) -- 91.8% chance to 2HKO after Leftovers recovery
Specially Defensive: Umbreon
This little black rascal is the Blissey of UU. It makes for great team support with moves like Heal Bell and Wish, and also has a good way of attacking opposing Pokemon.
Umbreon @ Leftovers
EVs: 248 HP / 252 SDef / 8 Def
- Heal Bell / Toxic
- Foul Play
Heal Bell and Wish are good for team support. Protect is to guarantee that you get the Wish healing, and can also be used for scouting. Toxic is an alternative to Heal Bell for 1v1 situations vs all except steel and poison types. Foul Play has great coverage, hitting type weak Pokemon, or those with large attack stats.
Physically Defensive: Gligar
Gligar equipped with an eviolite is truly the winner here allowing it to tank some incredible hits.
252 Atk Abomasnow Ice Shard vs. 248 HP / 252+ Def Eviolite Gligar: 100-120 (30 - 36%) -- 11.5% chance to 3HKO
It also has great typing that pairs well with Pokemon such as Umbreon.
Gligar @ Eviolite
EVs: 248 HP / 252 Def / 8 Spe
- Taunt / Toxic
- U-Turn / Stealth Rock
With a base speed of 85 this bat can outspeed a lot of the main walls in UU, such as Umbreon, Bronzong, and more, and get in a quick Taunt, or U-Turn to mess around with them. Earthquake is STAB and Taunt counter, capable of killing off some silly survivors. Toxic is good for 1v1 situations. Stealth Rock can be used if you don't already have a Rock setter. Roost is reliable recovery.
Those are the only tiers I'll do because I know very little about RU, and nothing about NU or LC. Doubles there really is no great wall, although Cresselia comes pretty close.
Hope this helps, and have fun battling!