260 Swampert

Mud Shot offers the highest energy generation in the game at the cost of low damage output. Due to how powerful Swampert's charge moves are, this move is recommended. Water Gun offers more damage output and is meant more for general coverage, but is not recommended due to its inferior energy generation. 

Hydro Cannon is a move limited to specific events such as Community Day, as well as Elite TMs, and is mandatory due to its lesser energy cost and very high Damage Per Energy value. It also completely blows Surf out of the water. Earthquake and Sludge Wave are both viable options, with Earthquake giving more power against Steel-types and Sludge Wave letting you hit Fairy-types harder.

Muddy Water is a very disappointing addition to Swampert for PvP. Hydro Cannon deals roughly 257% of the damage that Muddy Water can do, for only extra 5 energy. While Muddy Water does have a perk of reducing the opponent's ATK stage by 1, the 30% chance to proc the debuff makes it unreliable to use for consistent results. 

Arguably the best "mudboi" in the format, Mud Shot's speed combined with Hydro Cannon's power makes Swampert a massive threat, and one that any Grass-less team will struggle to answer. It's not that bulky, though, and it's massively vulnerable to anything with Razor Leaf, but a well-positioned Swampert is a fantastic choice to rain on your opponent's parade. Sludge Wave is more common than Earthquake here, giving you significant play against Azumarill (among other Fairy-types) and preventing you from getting walled out by Altaria.

Swampert is very good for Ultra League partly thanks to the numerous Steel-types in the meta, in addition to the relatively few Grass-types compared to Great League. Dominance over many of the "anti-Giratina" picks which are mostly Dark-types and also Snorlax, in addition to numerous wins against counters to those picks (AKA Fighting-types), makes Swampert very appealing to use. It should also be noted that Swampert can tie or even narrowly beat Giratina Altered Forme depending on movesets and shielding scenarios, which would possibly make this Pokemon a 5/5, if it were not for the lack of consistency in that matchup. Earthquake is preferable in Open Ultra to give you a strong neutral hit against Giratina, whereas both Earthquake and Sludge Wave are viable in Ultra Premier.

Swampert's relatively low max CP prevents it from maintaining its dominance from the other two formats. While its incredibly powerful moveset gives it some wins against common Steel-types in both Open Format and Premier versions of Masters, its general lack of stats and poor matchups against some incredibly meta Pokémon leaves the mudboi outclassed.