In-game, you don't really need a wall, as hyper offense is the way to go. However, I would say the best wall is Stakataka. Its Defense stat is monstrous, Its Special Defense is respectable, and its Attack is quite good. All these together make Stakataka serve as a solid wall, especially in-game.
Celesteela is more well-rounded in defenses and offense, and it could actually be considered a better wall than Stakataka. It could also serve adequately.
Guzzlord has the highest HP, at 223.
Kartana has the highest Attack, at 181.
Stakataka has the highest Defense, at 211.
Xurkitree has the highest Special Attack, at 173.
Nihilego has the highest Special Defense, at 131.
Pheromosa has the highest Speed, at 151.
Hope I helped!