0009 Blastoise

Water Gun performs better than Bite and also gets STAB. 

Behold, arguably the best charge move in the game: Hydro Cannon (40 Energy, 2.25 DPE). This is the main highlight of Blastoise, and is almost mandatory if you want Blastoise to perform at its peak potential. Hydro Pump is a lot slower and less efficient in comparison; if you're gonna use Blastoise then trade for one with Hydro Cannon, or don't use Blastoise at all. 

Ice Beam should be the second charge move for Blastoise as it provides meaningful coverage. Flash Cannon can hit hard, but with a 65 energy cost it's quite slow and doesn't really complement Blastoise anyways. 

Blastoise is one of the very few Pokemon blessed with arguably the best charge move in the game: Hydro Cannon. With a defensive stat distribution, Blastoise has the bulk needed to compete with a lot of meta picks in Great League. There are two main things that prevent Blastoise from being a premier Great League option. 1) it has mostly lackluster energy gain options for fast moves, limiting the use of charge moves including said busted Hydro Cannon  and 2) its inability to excel in any given role vs other options. Various other water mons (Azumarill, Lanturn, Quagsire etc) are often better at serving the role your team needs filled. This leaves Blastoise as either a budget replacement or as an option when you want to bring, say two water tanks, but those pesky rules keep you from bringing two Azumarills. Bonus swag points if you're rocking a shiny variant with shades from Community Day. #squirtlesquad 

Blastoise feels most comfortable in Ultra League, with a max CP of 2466. Hydro Cannon and good bulk still carry it to near the top of the ranks, though it gets shut down by the big Dragon-types and Grass-types, even with Ice Beam. Against any other opponent that doesn't resist Hydro Cannon, Blastoise is all set to blow them out of the water. 

Hydro Cannon isn't enough to carry Blastoise into Master League, where its lack of legendary stats severely hold it back from being anywhere near the top ranks. Wins against Groudon, Mamoswine, and Tyranitar are still appreciated though.