Moves
Sheer Cold is the big selling point of Articuno, allowing it to become an effective stallbreaker. Substitute protects Articuno from status moves and helps scout what Pokemon your opponent has chosen without forcing Articuno to switch on the next turn. Thanks to Articuno's bulk, most defensive Pokemon such as Cresselia, Suicune, Porygon2, and Slowbro will fail to break through its Substitutes. Roost ensures that Articuno can stick around, also allowing it to take Electric-type and weak Rock-type moves if it is faster than its foe.
Freeze-Dry gives Articuno some offensive presence outside of the Sheer Cold dice roll, allowing it to OHKO Garchomp and Mega Salamence, have a shot at 2HKOing Mega Gyarados, 2HKO most Dragon-types and Swampert, and deal good damage to Rotom-W. It's also good for saving Sheer Cold's PP when facing Suicune and Slowbro that are low on health. Alternatively, while Protect makes Articuno extremely reliant on Sheer Cold, it allows Articuno to effectively take advantage of Pressure to stall out Stone Edge from slower Pokemon with its Substitute and take advantage of timer stalling in emergencies. Hurricane has shaky accuracy, but it allows Articuno to OHKO Mega Heracross and have a shot at OHKOing Mega Blaziken switch-ins, in addition to letting Articuno hit Mega Venusaur hard without using valuable Sheer Cold PP.
Set Details
Articuno should always run Leftovers to set up Substitutes more often. 220 HP EVs maximize Leftovers recovery while letting Articuno switch into Stealth Rock twice without recovery. Maximum Special Defense lets Articuno tank plenty of special attacks, avoiding important 2HKOs such as defensive Rotom-W's Thunderbolt and offensive Mega Gengar's boosted Hex. Also, most defensive Pokemon that Articuno preys on use their Special Attack stat, while most of its checks are physical. 28 Speed EVs let Articuno outspeed most defensive Rotom-W and Suicune, while the rest go in Defense and Special Attack so EVs aren't wasted.
Alternate EV spreads are a viable option to consider if you are willing to give up on bulk. 220 HP / 148 SpD / 140 Spe allows Articuno to outspeed Adamant Breloom, while 220 HP / 52 SpD / 236 Spe beats Jolly Breloom. Finally, if you need to outspeed Jolly Gyarados and Dragonite, run a Timid nature with an EV spread of 220 HP / 60 SpD / 228 Spe.
Usage Tips
Articuno should be trying to prey on the defensive Pokemon of Battle Spot Singles, but you must note that directly switching into them risks getting Articuno crippled with Toxic poison or paralysis. Usually, it's best to come in via a slower U-turn or Volt Switch, bring in Articuno after a Pokemon of yours gets KOed, or even lead with Articuno. Once Articuno is safely in, use Substitute to block status. Most defensive Pokemon will be unable to break Articuno's Substitute, so use Sheer Cold as much as possible to apply a terrifying offensive pressure against your foes. If Articuno is carrying Protect, you can opt to whittle down the timer if you feel that your team is in a tough spot, because if the timer runs out and you have a higher number of Pokemon than the opponent, you'll win the match!
Against more offensive Pokemon that fail to 2HKO Articuno such as Mega Gengar, you should still use Sheer Cold as much as possible; however, don't bother using Substitute unless you intend to stall them out with Protect, as it's better to Roost in most cases. Articuno doesn't do very well against hyper offense teams in particular, so if you feel that many Pokemon on your opponent's team will just OHKO it with a move it's weak to, benching Articuno for the match might be the best course of action. This is especially true against Pokemon with Rock Blast such as Cloyster and Mega Heracross. However, note that Articuno's chances against offensive teams become much higher with paralysis support, since it can effectively use Substitute and Roost to stall out many Pokemon with weaker Rock-type attacks.
Team Options
Carrying Pokemon that resist Rock-type attacks is almost mandatory for Articuno, as it simply can't handle faster Rock-type attacks from the likes of Garchomp, Landorus-T, Breloom, Tyranitar, and Excadrill. Fire-, Electric-, and Steel-type attacks should also be covered if possible. Mamoswine is a great teammate to consider, because while it can't handle Steel-type attacks, it smashes through Garchomp and Landorus-T with Icicle Spear, has a shot of OHKOing Breloom if it gets at least three Icicle Spear hits, and hits Rock-, Steel-, and Fire-types hard with Earthquake. Bulky Landorus-T can take on most physical and Electric-type attacks aimed at Articuno and is your best bet against a Mega Heracross switch-in. Garchomp isn't as bulky as Landorus-T, but the two make for a fine offensive Ground-type pair that also resists Fire-type attacks and can use Choice Band Dual Chop to have a chance at defeating Breloom in emergencies. While Conkeldurr only resists Rock-type attacks, it's notable for being an excellent Tyranitar check and revenge killing Excadrill with Mach Punch. Mega Altaria is handy for dealing with Tyranitar, and being one of the few Pokemon to check both Charizard Mega Evolutions. Finally, Gliscor and Articuno can form a frightening OHKO attack duo, with Gliscor being chosen for teams with plenty of physical attackers while Articuno handles teams with dangerous special attackers.
Another avenue of support to consider is status. Paralysis in particular allows Articuno to set up Substitute more easily, Will-O-Wisp makes it much harder for most physical threats to check Articuno, and Toxic support means that Articuno won't have to rely as much on Sheer Cold. Porygon2 is an excellent teammate to consider, doing a great job spreading paralysis or Toxic and making a very good check to most offensive Ground-types and Aegislash that lack Substitute. Cresselia also works well, and while it can't check Aegislash, it can work as an emergency check against Mega Blaziken. Serperior is a more offensive twist on paralysis that shares an unfortunate Fire-type weakness with Articuno but makes a great Rotom-W check, gets a Speed boost from Rock Tomb, and can paralyze Ground-types with Glare. It can even use Reflect to make Articuno more difficult to check. Ferrothorn is even more weak to Fire-type attacks, but it makes a good status spreader and can even use Leech Seed to potentially restore more of Articuno's health. Rotom-H shares the dreaded Rock-type weakness, but it does great against Steel- and Electric-type Pokemon, except for Rotom-W, and it makes a good Mega Charizard Y check. It also is good at spreading around one of the three listed status moves if you wish. Heatran is a Fire-type that isn't weak to Rock-type attacks, and thanks to Flash Fire, it can switch into opposing Fire-types while spreading around Toxic and burns for Articuno. Mega Gengar is especially notable for its ability to trap and burn offensive Pokemon that Articuno has trouble with, and it also takes advantage of Pokemon with status via Hex.
When using Articuno on more balanced teams, it's a good idea to pair it with devastating sweepers that appreciate Articuno's stall-breaking power. Mega Kangaskhan is one of the most dangerous examples around, since it does very well against offensive teams. Mega Salamence shares Articuno's weakness to Rock, but it's very bulky otherwise, revenge kills Mega Heracross, and can switch into Fire-type attacks. Mega Blaziken appreciates Articuno's wallbreaking, smashes through Tyranitar and Excadrill (though it should watch out for Earthquake), revenge kills Mega Heracross, and can take on most Steel-types. You can even use a mixed Blaziken set to defeat non-Choice Scarf Landorus-T and Garchomp.