Say hello to Blissey, considered to be the best special wall throughout the generations. Sadly, with generation 5, Blissey's usefulness has diminished. Powerful special sweepers were brought into the metagame along with the introduction of Psyshock, meaning Blissey cannot be the special wall she once was. Keldeo, Alakazam, Latios, and Jirachi are all special attackers that Blissey just can't seem to handle. Blissey also faces stiff competition from Chansey, who has a lot more bulk whilst holding an Eviolite. Jirachi and Celebi also give Blissey competition as a special wall, as they have offensive presence and a much better typing. However, unlike Chansey, Blissey can hold Leftovers, allowing her to negate the residual damage from sand, hail, and entry hazards, making it much harder to wear her down. Her HP is the highest of any Pokemon and she can make use of it with massive Wishes that can heal more than 300 HP for her teammates. Blissey is considered to be the best cleric in OU, thanks to her fantastic support movepool and incredible special bulk. While Blissey isn't perfect, and is often setup bait for a lot of sweepers, she's usually the best cleric for weather stall teams, and must be considered when building a stall team.
Blissey is one of the best choices if your team needs a special wall. This is mainly because of Blissey's incredible HP stat and solid special defense, allowing it to sponge nearly any special attack. This set is often compared to Chansey, who is bulkier, but the key advantages to using Blissey are Leftovers recovery and a usable Special Attack. This means that Blissey is more suited to weather based stall teams, as having hail or sand damage chip away at your health is very unpleasant.
Wish allows Blissey to fully heal many Pokemon regardless of whether they invest in HP themselves thanks to Blissey's incredible HP stat. Protect should only be used with Wish, as it allows Blissey to receive the Wish and scout the opponent. Softboiled is another option to reduce Blissey's reliance on Wish for recovery, and is generally a better option if you're using Blissey as a cleric instead of a Wish passer.
Aromatherapy allows Blissey to heal her teammates and prevents Blissey from being Toxic stalled, allowing Blissey to further support the team. Toxic allows Blissey to wear down the opponent, and is Blissey's main way of damaging the opponent, which works well in conjunction with Protect. Thunder Wave has been slashed to better deal with fast sweepers like Venusaur and Volcarona while also statusing Fighting-types such as Lucario and Terrakion on the switch. Seismic Toss prevents Blissey from becoming complete setup bait by sweepers (especially most Substitute users trying to avoid Toxic), and allows her to reliably damage the opposition without worrying about her low offenses. However, Flamethrower hits most Steel-types immune to Toxic hard, such as Forretress, Scizor, and Ferrothorn, and makes Blissey a better counter to Venusaur. Keep in mind that it fails to guarantee a 2HKO on Ferrothorn and is less effective in a rain-filled metagame.Team Options & Additional Comments >>>
Stealth Rock is a move that literally every team must have, but there are so many Pokemon that learn it, so Blissey should rarely ever need it. Stealth Rock also allows you to stall out Gengar, but has little use outside of that. Blissey also has many other special attacks, but she really does not have the space for them, and they are pitifully weak for such a low uninvested Special Attack.
Blissey can use Calm Mind to boost her high Special Defense and her mediocre Special Attack, but she is still weak to physical attacks and is often forced out pretty quickly. Light Screen is an option, but is better left to Pokemon like Cresselia or Bronzong that have access to Reflect as well. Serene Grace increases the chances of secondary effects, like Ice Beam's chance to freeze, but Natural Cure is generally the better option because Blissey generally does not stay in for too many turns. Natural Cure also allows Blissey to recover from poison, which would otherwise cripple her. Counter and Gravity are less orthodox options, but can work due to Blissey's insane HP stat.
Specifically, Gravity also allows you to use more powerful moves on Blissey, such as Thunder, Blizzard, and Fire Blast, but it's gimmicky at best. Skill Swap can be used to stop Reuniclus from destroying your team, but Blissey is hit hard by Psyshock. Psych Up has some value in that it can go through Substitutes and this steals defensive boosts like a Quiver Dance or a Calm Mind boost from a Jirachi, Reuniclus, or Celebi, but it has little use outside of that. Blissey has some gimmicky options in Defense Curl, Sing, and Charm, but Blissey has better things to do.
Checks and Counters
The large list of Fighting-types in OU are the best way to keep Blissey in check; Terrakion, Breloom, Keldeo, Infernape, Conkeldurr, and Lucario are all Pokemon that can switch in pretty easily and force Blissey out. However, they must be wary of Thunder Wave. Any strong physical attacker with high-powered STAB attacks or Fighting-type coverage can muscle past Blissey. Kyurem-B has a titanic base 170 Attack and a strong STAB in the form of Outage, while Blissey can't scratch it back. Strong Ground- and Dragon-types like Garchomp, Choice Band Dragonite, and Double Dance Landorus-T can easily get past Blissey or force it out.
Jirachi is immune to Toxic and can set up 101 HP Substitutes to allow it to get multiple Calm Minds in without getting worn down by Seismic Toss. It can also paralyze Blissey and flinch her down with Iron Head, or even trick a Choice Scarf onto her. Other bulky Calm Mind sweepers like Reuniclus also give it hell thanks to Magic Guard and Recover. Stallbreakers like Gliscor, Reuniclus, and Gengar easily force Blissey to switch. Gliscor is immune to status thanks to Poison Heal and can Taunt Blissey due to its higher base Speed. Reuniclus has Magic Guard and can use Blissey as setup bait. Gengar is immune to Toxic and Seismic Toss and can use Blissey's own health to its advantage via Pain Split and can also carry Taunt.
While Blissey used to take on most Psychic-types easily, with the release of Psyshock, it's a much different story. Latios, Alakazam, and Reuniclus can get past Blissey easily with this attack, as it attacks Blissey on her much lower physical defense. Other Trick users, like Latios and Rotom-W, cripple Blissey by replacing her beloved Leftovers with a handicap.