Jirachi

Great League: 3.5 / 5

Jirachi was initially set to become a dominating force in PvP thanks to what would have been the best charge move in the game: Doom Desire. A slight nerf to Doom Desire's PvP stats pre-release hurts Jirachi a bit, as it requires an additional fast move to build enough energy. Unfortunately Jirachi now loses to many of the popular Water-type contenders like Whiscash and Blastoise, in addition to just getting walled by Dark-types like Umbreon and Sableye. Because Jirachi doesn't have enough bulk to tank Super Effective attacks for long, Jirachi tends to do better if it can either shield itself twice, or fight with a shield advantage. Despite this, Jirachi is still quite good since Doom Desire is comparable to the Community Day moves that are exclusive to starter Pokemon. Overall a really good pick for Great League, and if anything definitely worth trying. 

Ultra League: 3.5 / 5

The pre-release nerf of Doom Desire is a much bigger wound in Ultra League, as Jirachi was reliant on more frequently abusing Doom Desire to take on much of the meta. Regardless, Jirachi is still considerably good in Ultra League since it can shut down most of the Fighting-types that help cover Giratina, as well as straight up winning against Giratina Altered Forme itself if there's no Ghost-type moves in play. Similarly to Great League however, Jirachi is held back by unextraordinary bulk which means it typically prefers being shielded twice, or at the least to fight with a shield advantage. Overall Jirachi does a really good job in Ultra League, but in the big mess of Giratina, anti-Giratina, and the other complexities involved, Jirachi needs to pick its fights carefully, especially against any opponent with a Ghost or Dark-type moveset. 

Master League: 3 / 5

Jirachi has a max CP of 3265, which is somewhat behind that of many other legendary Pokemon in Master League. Jirachi's powerful moveset is what allows it to get wins off the more intimidating titans like Dialga. Just like in the other leagues, Jirachi performs better with double shields, but now loses more easily to opponents with a typing or moveset advantage. Overall a really good pick for PvP, being capable of walling the popular Dragons not named Giratina Origin Forme, and fighting back against Groudon and Lucario with double shields. While the Stardust and Rare Candy costs to max out Jirachi might be quite high, Jirachi for the most part doesn't actually need a second move as long as it can abuse Doom Desire.