Suicune is arguably one of the best Pokemon in the game. It is almost impossible to KO and even harder to stop once it gets rolling with Calm Mind and its high defenses. When used properly, it can sweep entire teams with ease.
This is the classic Suicune set. Once it switches in, Suicune can usually start setting up Calm Mind right away. With a few Calm Mind boosts under its belt, it can proceed to attack with Surf. The last moves are all interchangeable but all function very differently. Sleep Talk allows Suicune to set up against most Pokemon and continue to be a threat, even while sleeping. You should definitely consider this option for a team that revolves around Suicune, as it keeps any Pokemon that may think about trying to dent it while sleeping at bay. It also gives Suicune an easier time against Zapdos, provided Zapdos doesn't have Roar and Suicune has a few Calm Minds under its belt. Roar wins out against other Calm Mind users (including other Suicune!) and, when paired with Spikes, can keep forcing out Suicune's most common switch-ins: Blissey, Celebi, and Raikou. Roar also acts as a last ditch attempt to stop any physical Pokemon who got one too many boosts against you. Finally, Ice Beam plays a direct role, allowing Suicune to effectively check Salamence and Flygon throughout the game, while also beating Zapdos and Celebi much easier. If the opponent tries to stall Suicune out, Ice Beam can freeze their counters.
Maximize Suicune's HP and Defense in order for it to take on Tyranitar, Aerodactyl, Dugtrio, Metagross, and other physical powerhouses. If Choice Band Tyranitar is constantly ruining your day, consider raising Suicune's Speed to 222 with 64 Speed EVs. This will outspeed any Adamant Tyranitar and let you take them out. It also gives Suicune the jump on other Suicune if both are using Roar.
Unlike SuperCune, this set uses some of Suicune's most common counters as set-up fodder. Investing in max HP prevents Blissey from ever breaking Suicune's Substitute with Seismic Toss, allowing it to set up Calm Mind. If sandstorm isn't active, Suicune can actually set up all of its Calm Minds and keep a Substitute intact, while still killing Blissey. Substitute also blocks Toxic and Leech Seed from bulky Water-types and Celebi, respectively. Surf is the usual attack, but Hydro Pump provides more immediate power for threats like Snorlax. The PP and accuracy of Surf usually makes it the better option.
Use 64 Speed EVs to beat max Speed Tyranitar to the punch. You can run more Speed to set up on slower Zapdos and Celebi more easily, but it's hard to gauge just how much Speed either of them will be running.
Suicune can simply function as an excellent wall without Calm Mind. Sleep Talk allows Suicune to shuffle through Roar and Surf, racking up Spikes damage on its most common switch-ins, while still posing a threat to Tyranitar, Aerodactyl, and others. The threat of Sleep Talk selecting Roar is what separates this from most bulky Water-types, and is why this Suicune has become more popular. While you won't sweep many teams with this variant of Suicune, you'll be able to effectively wall most of the metagame, as Roar will constantly be bouncing anything that attempts to set up on you. Just be careful of being set-up fodder for opposing Spikers, as Suicune has no real way to threaten them.
With Suicune's fantastic Defense, it can easily switch in and set up Reflect to support its teammates. Mirror Coat can surprise Electric-types hoping to deal heavy damage with Thunderbolt. Toxic can be used to wear down last Pokemon Blissey or to Rest stall some of Suicune's counters. It's decent, but generally just doesn't have room on Suicune's standard sets.
Checks and Counters
The best counter to Suicune is Celebi, who can Leech Seed, Calm Mind alongside it, and even use Perish Song to force it out. Suicune can hit it with Ice Beam, but Celebi's presence is enough to usually force Suicune to sit around for much of the game. Calm Mind Jirachi can set up Calm Minds alongside Suicune and blast it with Thunderbolt, but must be cautious of Roar or critical hits. Blissey is yet another Calm Mind user who can set up, but defensive Blissey will struggle to get going. Regice can Psych Up boosts and use Thunderbolt to force Suicune out. Milotic can temporarily put Suicune out of commission with Toxic or Hypnosis, which allows another sweeper to set up on it or just go for the KO. Gyarados is a solid answer thanks to its decent natural Special Defense. It also has Taunt at its disposal to prevent Suicune from using Roar; it can set up Dragon Dance boosts at will and sweep.
Suicune is highly vulnerable when asleep if it lacks Sleep Talk; hard-hitters, such as Choice Band Salamence, can KO it before it wakes up. Most powerful Choice Band users work similarly—with the exception of Metagross, as Suicune resists Meteor Mash—but Metagross's Earthquake isn't much weaker than Salamence's Hidden Power Flying. Heracross can take it on any time whether it's awake or asleep, but a sleeping Suicune is much easier to switch in on. Choice Band Megahorn 2HKOes Suicune, and Heracross also has decent Special Defense to take a few Surfs. Heracross should be cautious of more offensive versions of Suicune.
Zapdos can take Suicune on with STAB Thunderbolt, but it doesn't like switching into Ice Beam. Raikou fares a bit better, as it has better Special Defense and isn't weak to Ice or Water; it can also set up Calm Mind against a sleeping Suicune or one that lacks Roar, and sweep its whole team. Even Roar variants will often be too afraid to stay in and phaze for fear of taking ~70% from Thunderbolt, and even if they do, Raikou can just come right back in and try it again. If you wait too long, however, Suicune can get some Calm Mind boosts in, and with as little as one Calm Mind, that 2HKO can become a 3HKO, and Suicune's 3HKO on the counter can be a 2HKO. This is why physical threats such as Heracross are such good counters compared to Electric-types: even if Suicune gets several Calm Mind boosts, Heracross can take it out with impunity. Of course, immediate response to Suicune in the form of a hard-hitter will also stop this. While Tyranitar itself loses handily to Suicune, its Sand Stream ability puts a damper on Suicune's tanking ability, as the latter lacks a recovery move outside of Rest.