For me, I'd have to say its between Overcoat Mandibuzz, Prankster Sableye and my newest addition in Ferrothorn. I didn't like using Ferrothorn originally because of its overuse. Nowadays I haven't seen a seed of it as much. I actually used an odd combination, that works with its overpowered defenses. It's a Subset so it can use its Support role to stun Fire Types. It's a 3 in 1 pokemon, Striking most pokemon, Walling most moves, and supporting its team and it does it well.
Mandibuzz is my favourite wall of them all. It is designed to be a bulky troll, even taking super effective moves for at most a 3 hit KO funnily enough. Due to the overuse/superior high chance of males appearing in battle, Mandibuzz is pretty much a beast. A friend who prefers Braviary over it even said that out of all the Pokemon I used against him, that birdy is my best one. It won't be good against most legendaries (due to them being Genderless) but I love that bird and trust it with my soul. Mandibuzz is more focused as a Supporter then a Wall.
Prankster Sableye is one I've been trying to make work for ages but thanks to a friend's help, I got one. It's not much a wall but it is more a pest then a fighter under its status as a Supporter. Sableye is pretty much the scout of the team thanks to it's higher priority.