Here I will try to list all notable OU walls, and this is one of 2 or more answers because maximum limit is 1200 characters
Rain is an extremely popular playstyle in the metagame, with abusers like Urshifu-R, Kingdra, Zapdos, and others. Ferrothorn has the ability Iron Barbs to punish physical Water type moves like Flip Turn, Waterfall, Liquidation, and Surging Strikes. Not only that, but Ferrothorn is very sturdy, and has a excellent defensive typing. However, Fire type moves usually destroy Ferrothorn thanks to its 4x weakness. Rain halves the power of Fire type moves though, making it even harder to break past. The amount of switches it forces lets it set up hazards, leech seed switch-ins, Knock and cripple, paralyze with Thunder Wave, or obliterate the Pokemon coming in with its powerful STAB moves in Gyro Ball and Power Whip. Ferrothorn is also immune to Sandstorm and resistant to Stealth Rock.
Its no surprise Toxapex is here. 50/152/142 defenses paired with resistances to Fire Blast, Scald, Ice Beam, Focus Blast, Close Combat, Sludge Bomb, U-Turn, Iron Head, and Moonblast let it come in for basically free against many offensive threats and proceed to use its vast support movepool to great effect. Toxic Spikes, Scald, Haze, Recover, Knock Off, and Baneful Bunker. Most super-effective moves don't even do more than 50% to Pex, and the ones that do, it can just Recover or switch out, since it has one of the best defensive abilities in the game: Regenerator, which heals 33% of health when you switch out. It is also immune to Toxic and Scald burns from opposing Toxapex don't out-damage Black Sludge. Speaking of Black Sludge, Pex's half-poison typing allows it to use this item effectively. Since Black Sludge damages any Pokemon that isn't Poison type and heals Toxapex the same amount Leftovers does, it can really mess with Trick users and stack damage. It makes for an excellent pairing with Ferrothorn, since Ferrothorn resists Electric and Psychic and is neutral to ground, and Ferrothorn is weak to Fighting and Fire, which Toxapex resists.
Ever since Heavy-Duty Boots were released, Blissey has been taking over the metagame. That huge HP stat, the highest of any Pokemon, as well as a great Special Defense stat, and (technically) an immunity to Toxic and Will-O-Wisp thanks to Natural Cure, as well as an immunity to Stealth Rock, Toxic Spikes, and Spikes because of Heavy-Duty Boots have cemented Blissey's place as one of the best walls around. It also has an incredible movepool, with Aromatherapy/Heal Bell, Teleport, Soft-Boiled, Toxic, Seismic Toss, and Stealth Rock. Its access to Wish however is what truly sets it apart. While not compatible with Teleport, Wish's mechanics were changed in Gen 5 so that the reciever is healed by an amount equal to half the Wish user's HP, so if a Blissey with 31 HP IVs and 252 HP EVs used Wish, the Pokemon recieving the Wish would get 357 HP back. Despite Blissey's base 10 Defense stat, that massive HP makes it much physically bulkier than it looks like on paper. With Urshifu-S being suspect tested, if it gets banned, Blissey will almost without a doubt rule the metagame.
Right now, however, the best Pokemon in the metagame is Clefable, and for good reason. It has a spammable STAB move with 24 PP, which is 95 Base Power, a 30% chance to lower foe's SpA, and a lot of super-effective targets. It also has one of the biggest movepools in the entire game, with great support moves too (more on that later). Mono-fairy is incredible defensively, with resistances to Wicked Blow, Dark Pulse, Focus Blast, Close Combat, U-Turn and an immunity to Outrage, Scale Shot, Draco Meteor, and Dragon Claw. It also has 2 INCREDIBLE abilities in Magic Guard and Unaware; the former makes it immune to Sandstorm, Toxic, Will-O-Wisp, entry hazards, and Leech Seed, while the latter lets it ignore Swords Dance Garchomp, Bulk Up Cinderace lacking Gunk Shot, Bulk Up Corviknight, Calm Mind Clefable, Dragon Dance Dragonite, Dragon Dance Dragapult, Swords Dance Excadrill, Misty Seed/Swords Dance Hawlucha, Nasty Plot Hydreigon, Swords Dance Landorus-Therian, Calm Mind Magearna or Calm Mind Magearna, Swords Dance Rillaboom, Nasty Plot Spectrier, Dragon Dance Tyranitar, Bulk Up Urshifu-S and Bulk Up Urshifu-R, Quiver Dance Volcarona, and a whole bunch of others. It also has very well-balanced bulk, with 95/73/90. Now, back to that movepool. Wish + Teleport + Magic Guard is broken. Clefable can ignore Stealth Rock and Toxic Spikes and pass 194 HP Wishes to a teammate.
With 110/105/95 defenses, Mandibuzz is one of OU's premier walls. Before, Buzz wasn't a top-tier OU wall, largely because of its Stealth Rock weakness, but Heavy-Duty Boots fixes that. Mandibuzz has good support and longevity moves like Knock Off, Whirlwind, Defog, Roost, Foul Play, U-turn, Taunt, and Rock Tomb. It also has a pretty good Speed stat for a wall. Its ability Overcoat makes Mandibuzz immune to Sandstorm, Spore, and Sleep Powder.
Buzzwole's typing and movepool make it a good physical wall. STAB Drain Punch to heal you, Roost for more consistent recovery, Bulk Up can be used to raise Defense and boost the power from those Punches, and by Punches I also mean Ice Punch which does heavy damage to Landorus-T and Garchomp. An Assault Vest set is also plausible, since Buzzwole's base 53 Special Defense stat does need improvement, even with a base HP stat of 103.
