Registeel: Massive base Defenses (BOTH) at 150. Steel typing nullifies Counter-Intoxication.
Porygon2: An Eviolite makes this thing a deadly wall OR Tank. It also has Recover available to it to keep it alive.
Mandibuzz: Has the Overcoat Ability available to effectively use in weather. Also has Roost and very good HP, good Defenses and decent Special Defense.
Blissey: I know it has horrible Physical Defenses, but Counter can make up for a neutral hit for massive damage. It's got the most HP any Pokemon can have. (Base 255 !!!) You could also try lowering their attack power with Charm, and it also has high Special Defense. Also works well with Pain Split, but I have never known how Pain Split works.
Chansey: Read above. A lower-tier version. Still has massive HP. Forgot to mention it also has Softboiled.
Shuckle: Everyone has to agree he's built for some serious stalling. Base 5 offensive stats and base 235 defenses.....My god. Stalling MACHINE. Toxic and Wrap work well on him.
Jellicent: This is a very good Special Wall. A nice 105 Sp. Def paired with the 100 HP and Toxic + Recover + Scald (30% burn rate with the burn lowering physical vulnerability) + Substitute + Lefties make a terrifying sea monster.
Cofagrgus: Huge Defense paired with good Special Defense makes one hell of a staller. He's a fave of mine. Lefties, Protect, Calm Mind, Hex, and Toxic make a great Cofagrigus.
Deoxys-D: How could I have forgotten this one? It's the top-rated defensive legendary! It has base 160 base stats in both defenses, making a max of 414 neutral-nature wise. But he has a large weakness- his base HP is only 50, so be careful. It does, however, have Recover to back that up. Unfortunately, as of March 1st, 2013, Deoxys-D is banned. That sucks a lot.
Regirock: Awesome Physical Wall, but it needs to be careful with the special attacks. It's got a big Defense at base 200, out-defensing even Registeel. But the catch is that he only has base 100 Sp. Def. and base 80 HP. It always has to have Curse, so it can go overkill later with Drain Punch or a STAB move.
FEAR Aron: http://www.youtube.com/watch?v=oJxvmtCwAZY
Walrein: It's got Ice Body, Leftovers, Aqua Ring, Hail, Toxic, and Protect. What more could you want for an NU wall?
Blastoise: Same thing as Walrein but with Rain Dance and Rain Dish. And it's UU.
Ok now I've come to the point where I can't think of any more. Comment for more.