Upon taking a quick look over Ariados's stats and typing, one might wonder why you would ever want to use this spider. Indeed, there are many negative things to say about the arachnid; his typing is generic and provides him with a Stealth Rock weakness, and isn't great offensively either. There are many other Pokemon that give him competition for a slot on your team depending on his moveset, such as Garbodor and Pinsir. As if that wasn't enough, Ariados's stats leave much to be desired, with his only usable stat being his decent base 90 Attack, and his base 40 Speed being particularly awful. Ariados also got shafted with the new generation—he can no longer fulfill his role of Baton Passing Spider Web due to the new mechanics in BW. Despite this, Ariados is still able to fill a niche role in NU: that of an offensive Toxic Spikes user. Although his typing is bad offensively, it does provide him with 4x resistances to Fighting- and Grass-type moves, which gives him a window to switch in easily. Overall, although one cannot call Ariados an exceptional Pokemon, he is still usable.
Toxic Spikes is Ariados's best option for a set that isn't completely outclassed. Toxic Spikes are very lethal in NU due to the lack of good Poison-types, although Pokemon such as Vileplume and Skuntank are common and absorb them. One of the things that sets Ariados apart from Garbodor and other Toxic Spikes users is his access to decently strong priority moves, which allow him to get a strong hit on Pokemon such Charizard, who would otherwise KO him. Sucker Punch hits twice as hard as Shadow Sneak, but it relies on prediction and can leave Ariados prone to being set up on; as such, Shadow Sneak can be used instead.
Poison Jab is Ariados's strongest STAB move and enables him to hit many Pokemon for a decent amount of damage. Although Bug Bite has a lower Base Power than Poison Jab, it has much better super effective coverage, such as against common Psychic- and Dark-types, including Gardevoir. Poison Jab is generally preferred, however, as its higher Base Power comes in handy against Pokemon who take neutral damage from it, such as Miltank and Charizard. Additionally, Poison Jab has better neutral coverage against threats such as Swellow, Charizard, and Rotom-S. Protect is chosen pretty much for lack of a better move to use. Nevertheless, it prevents the rare Fake Out lead—such as Persian—from breaking Ariados's Focus Sash, and also allows him to scout moves, especially from Pokemon holding Choice items.Team Options & Additional Comments >>>
Ariados cannot viably run many other sets. A Baton Pass set was viable last generation, but Ariados lost his ability to Baton Pass Spider Web, meaning that he can only pass Agility boosts. However, Ariados's low Speed and bad bulk make him greatly outclassed in this role by the likes of Gorebyss, Lopunny, and Emolga. Ariados's access to Disable enables him to run a Substitute + Disable set, but his abysmal Speed makes it terrible in reality.
Night Shade is also an option, providing a supply of constant damage. It also means that Ariados will not have to run Attack EVs, allowing him to invest in his bulk some more. A purely physical attacking set could work, but the only usable physical options that Ariados gets in addition to his STAB moves are Sucker Punch, Shadow Sneak, Return, and Pursuit, granting him ridiculously poor coverage: no matter what moves Ariados uses, he will always be walled by Steel-types such as Probopass, as well as any remotely physically bulky Pokemon. Regirock and Golem in particular completely counter him very well, as they can easily shrug off any of his attacks and hit him hard with STAB super effective Rock-type attacks.
Lastly, one might wish to use one of Ariados's other abilities. Swarm provides a useful power boost to his Bug Bite at low health, but nearly everything is strong enough to KO Ariados when his HP is low enough for Swarm to kick in. Sniper on the other hand is fantastically gimmicky and luck-reliant, as Ariados will be sure to cause major damage 6.25% of the time. In general, however, you would be better off sticking with Insomnia, which allows Ariados to more easily switch into Tangela and other Sleep inducers.
Checks and Counters
Unfortunately for Ariados, counters to him are very numerous. Grounded Poison-types, mainly Vileplume and Skuntank, are the bane of Ariados, as they can suck up the Toxic Spikes he worked so hard to set up merely upon switching in, meaning they cannot be stopped. As mentioned, Taunt users such as Misdreavus completely put a halt to Ariados's shenanigans. Most faster Pokemon are able to at least 2HKO Ariados, limiting him to only a single layer of Toxic Spikes.
Rock-types also easily beat Ariados by tanking his weak attacks and KOing it. Particularly problematic are those that commonly run Rock Blast, such as Golem and Gigalith, as they can strike right through his Focus Sash. In fact, most Pokemon with multi-hit moves other than Bullet Seed or Arm Thrust can beat Ariados. Most Ground-types such as Golem and Camerupt can easily beat him due to their resistance to Poison Jab, with Camerupt also packing a resistance to Bug Bite. Finally, Marowak can set up a Substitute on Ariados before using Bonemerang to get rid of the poor spider.