Unfortunately Arbok hasn't really stood the test of time very well. It is outclassed by other Poison-type Pokemon that were luckier in terms of their stat distribution -- Arbok's main downfall is that it is very underwhelming stat-wise. Its base 95 Attack is alright, but nothing impressive, and beyond that it is a bit frail and can't do very much with its middling base 80 Speed. Even in Smogon's lowest tier PU, Arbok only manages a usage rate of 1.3% and just 0.2% from players who are high on the ladder, which I think is telling that it has turned out a bit of a dud.
If you're really interesting in testing its abilities in competitive though, Arbok has some redeeming qualities that might give it some fun moments. Its pure Poison typing has three more resistances than it does weaknesses, including helpful switch-in opportunities to common Fighting and Fairy attacks. Its ability to switch-in is improved by its ability Intimidate, which should give it respectable physical bulk for an offensively inclined Pokemon. It also has a good movepool including options like Gunk Shot, Earthquake, Sucker Punch and Aqua Tail.
Arbok's most viable set by far makes use of Coil, which gives it the jump on its power required to secure KOs and makes Gunk Shot far more reliable. Coil also capitalises on some of the switches that Arbok could force using its good defensive typing and movepool. On this set, Waterium Z and Aqua Tail could allow it to surprise and muscle past Ground-type Pokemon, whilst Black Sludge is a more consistent option otherwise. Sucker Punch could allow it to play against Psychic-type Pokemon and make up somewhat for its Speed.
Really though, there isn't much of a reason to be using Arbok seriously. Other Poison-type Pokemon like Alolan Muk pull off the slow set-up sweeper set better and have more interesting stats, whilst Pokemon like Toxapex do better in a defensive or stall role as well. If you do use Arbok, use it because you enjoy it.
