395 Empoleon

Waterfall is preferred for its high damage output, which is further boosted by STAB. 

Hydro Cannon is one of the most powerful charge moves in the game, and is necessary not only for its high DPE value, but also due to its significantly less energy cost. Drill Peck ties for the cheapest charge move energy-wise, which is important given Empoleon's poor energy generation. It also provides useful coverage against its vulnerability to Fighting-type Pokemon. Flash Cannon benefits from STAB and is cheaper than Blizzard, but the latter provides significantly more valuable coverage. Neither of these two moves are recommended due to their expensive energy costs. 

Empoleon is a Water/Steel type Pokemon with average bulk, and access to the powerful Hydro Cannon. Water Pokemon are widely used in the meta, and while Empoleon competes with many other Water-types for a roster slot, it stands out with its unique Steel sub-typing which adds a plethora of resistances. Relatively poor energy generation holds it back however, as it limits the frequency of which it can use Hydro Cannon or its coverage options. 

Empoleon struggles in Ultra League due to common anti-Steel Pokemon which are utilized in the meta. Water-typing offensively is also not as valuable in this particular meta, which puts Empoleon in a difficult position. It should be noted that Empoleon performs better in specific formats like Premier Cup, where it doesn't have to worry about its loses to certain legendaries like Giratina and Registeel. Access to Drill Peck is useful against Fighting-types and Grass-types, but might not be enough to win under most conditions due to poor energy gains and the lack of STAB. 

Empoleon lacks sufficient stats to perform well in Master League. Its relatively cumbersome moveset also sets it back from Water-type powerhouses, which outperform Empoleon through raw stats or faster movesets.