537 Seismitoad

Bubble features considerably more damage output than Mud Shot, however the latter generates energy faster. Either option can work depending on how you want to use Seismitoad. 

Earth Power is Seismitoad's strongest charge move, and provides wide coverage against common meta Pokemon. Muddy Water has pitiful damage output, but costs much less energy and is used primarily to bait shields and/or debuff opponents. Sludge Bomb can be elected for coverage against Seismitoad's weakness to Grass-type Pokemon, and can also hit Flying-types for some decent damage. 

Seismitoad sports a Ground/Water typing with average bulk, which puts it into comparison with other popular Pokemon nicknamed "mud bois" based their Ground/Water typing. Seismitoad stands out with its access to Earth Power, which offers a middle ground between low energy Mud Bomb and hard-hitting Earthquake. However the lack of a significant secondary charge move makes Seismitoad somewhat of a one-trick-pony, and leaves it quite vulnerable to Flying-types in particular, or when Ground-typing would be resisted otherwise. 

Access to a powerful Ground-type charge move is a very appreciated asset in Ultra League due to the prevalence of Steel-type Pokemon. Fast energy from Mud Shot in particular allows Seismitoad to threaten Earth Power frequently which gives it numerous wins even in seemingly neutral fights. However, it's quick lackluster against counter picks, and loses to Giratina. 

With a max CP of 2564, Seismitoad lacks sufficient stats to perform well in Master League. Even though its moveset is effective against Steel, meta Pokemon in this league often have enough stats to overcome this typing disadvantage.