Suicune is without a doubt a highly accomplished member of the second legendary trio in the game. It has great defensive stats and a good HP stat, which means that it can perform well as a very bulky Water-type. Offensively it's not a bad Pokemon either; it has a base 90 Special Attack and a decent base 85 Speed. It also has access to Calm Mind to further boost its Special Attack and Special Defense, letting it perform well as either a tank or a sweeper.
However, time hasn't been kind to Suicune. The legendary dog has had increasing troubles since its first appearance in Johto; now, with the significant increase in power of the offensive threats, its defenses aren't as amazing as they used to be and it now has a hard time taking even some unboosted hits. Suicune also has a somewhat shallow movepool, which makes it predictable. However, despite these two limitations, it is still a top Pokemon that one should always consider when building a team.
Calm Mind is the main move here as it allows Suicune to boost its already impressive Special Defense and decent Special Attack and start a sweep. Surf is a reliable and powerful STAB attack that Suicune loves to spam after a few boosts. Ice Beam allows Suicune to hit the Grass- and Dragon-type Pokemon that resist its STAB attack for major damage. The last slot represents the most difficult decision and depends on Suicune's teammates; Hidden Power Electric is usually the best choice as it lets Suicune get past bulky Water-types that would otherwise stall out the legendary dog or defeat it with Toxic.Team Options & Additional Comments >>>
This is a different approach to fulfilling Suicune's potential; it trades immediate power and Speed for huge bulk and great durability. It may have an unappealing Special Attack at first but it has enough bulk so that it can easily find an opportunity to set up Calm Minds, turning it into a monster. Scald is the only attacking move, but this is acceptable thanks to the great neutral coverage Water-type moves provide. The burning chance may also provide Suicune with an easier set-up on physical attackers, and may help wear down the opponent's Pokemon. Calm Mind even allows Suicune to set up on some super effective special attacks if it manages to get a boost on the switch-in. Rest and Sleep Talk are the moves that protect Suicune from status and allow it to recover the health it lost while setting up.Team Options & Additional Comments >>>
This set attempts to cause some major damage with little set-up. Calm Mind is the boosting move of choice and turns Suicune into a bulky tank after some turns. Substitute is this set's main selling point; it provides Suicune with a status immunity and works very well with Calm Mind as, with Suicune's natural bulk, its Substitutes won't be broken even with super effective attacks after a few boosts. Surf is the powerful and reliable STAB attack of choice, while Ice Beam provides good coverage against the Grass- and Dragon-type Pokemon that resist the Water-type attacks.Team Options & Additional Comments >>>
Outside of the listed sets, Suicune doesn't have many options. Toxic is a good move to hit the bulky Waters that trouble the legendary dog so much and can be used instead of Toxic Spikes to get past the aforementioned Pokemon. In a metagame filled with Fighting-type Pokemon, Extrasensory can be used to hit them super effectively but STAB Surf or Hydro Pump already deals major damage to most of them. Shadow Ball is not really worthy of use for similar reasons.
Suicune has major bulk and access to Rest so it can work as a bulky Rain Dance user. With defensive investment it can take lots of hits but shares weaknesses with common rain abusers, making Uxie or Scizor a much better choice.
Checks and Counters
Bulky Water-type Pokemon are Suicune's biggest counters as they resist its STAB attacks and have ways to prevent a sweep: Jellicent may have Taunt or Toxic, both of which ruin Suicune unless it is running the Substitute + Calm Mind set and even that requires Toxic Spikes support to beat the jellyfish; Vaporeon has Water Absorb to protect itself from Suicune's STAB attacks and Roar to force it out but it must watch out for the Offensive Calm Mind set as +1 Hidden Power Electric is a 2HKO; Gyarados has to watch out for Hidden Power Electric but if Suicune doesn't carry it, it can phaze Suicune to prevent a sweep.
Ferrothorn is another major obstacle to the legendary dog as it resists every move Suicune has barring the rare Hidden Power Fire, and can use Power Whip and Leech Seed to damage the legendary dog. If Suicune doesn't have Hidden Power Fire, its only chance to protect itself from Ferrothorn is Scald's 30% burn chance. In fact, if Suicune can burn Ferrothorn, Power Whip will only do around 25% so it is no longer impossible to get past.
Chansey and Blissey appear as the primary counters for the offensive versions of Suicune as they can come in, sponge any special attack it throws at them and use Seismic Toss to KO. However, the pink blobs have to watch out for the defensive versions as they may become setup fodder.
Suicune's Dream World ability is Water Absorb and, even though Suicune doesn't take significant damage from Water-type attacks due to its great bulk, it's not a useless ability. The defensive sets are better paired with Pressure as Suicune will be on the field for a while and can PP stall some strong attacks that have low PP. However, the Offensive Calm Mind set lacks recovery and, as such, can make use of Water Absorb to recover some health before attempting a sweep.