Toxapex has incredible defense (152) and special defense (142), but is slightly nerfed by it's low HP (50).
It's poison typing means it not only removes toxic spikes, but is also immune to toxic, a common move used to cripple walls. It also means it's weak to common types like ground (earthquake) and psychic (psychic, psyshock)
Toxapex's incredible ability, regenerator, is possibly the best ability for a wall in the game, as it heals 1/3 of it's health whenever it switches out.
As for moves, Toxapex gets tons of fantastic moves. Most notable are:
Haze to deal with Pokemon who try to setup on it.
Scald to cripple physical attackers with a burn.
Toxic to semi-stall out opposing Pokemon.
Toxic Spikes to lay down valuable hazards.
Recover to reliably heal health.
Despite seeming nearly perfect, it does still have some counters. Psychic types can pretty easily handle toxapex (though some like Mega Galade do not appreciate burn), Zapdos can defog away toxic spikes and deal hard-hitting thunderbolts but just HATES toxic variants, Ground types give it some trouble but they are often weak to scald and none can take a burn very well, and taunt can successfully shut down Toxapex easily, leaving it to spam scald or simply switch out.
All in all, Toxapex's ability to successfully wall with it's high defenses and immunity to toxic as well as it's options fight back against it's own counters makes it a very successful (and popular) wall.