Blastoise

Blastoise: the OG bad Vaporeon. After so many generations of Pokemon, there are now numerous bad Vaporeon, who itself is now a worse Gyarados, who itself is a worse Kyogre, etc. Even on defense, Blastoise is also a bit underwhelming. Its fast moves barely do anything to attackers, while its bulk is nowhere close to that of Vaporeon or Milotic.

As a Dynamax Pokemon, Blastoise is pretty solid. It's got good Water Type damage, but it falls behind Inteleon with Max Geyser, and isn't the best option when fighting against Psychic or Ghost Types with Max Darkness. Also, note that it's going to be outclassed by itself on the Water front when Gigantamax Blastoise drops.

Best PvE Offensive Moveset

• Water Gun has the advantage of Water Type STAB and better overall performance, putting it in a stronger position than Bite.

• Hydro Cannon* is objectively outstanding. Don't bother with other options.

• Hydro Pump is Blastoise's optimal non-exclusive charge move.

• Ice Beam has its select advantageous matchups.

• Flash Cannon is an awful 1-bar charge move, and Skull Bash isn't useful either.
• Return offers nothing of any value.

Best PvE Defensive Moveset

• Water Gun hits a little harder than Bite thanks to STAB and also gains energy faster, but Bite has wide neutral coverage that leaves Blastoise less of a sitting duck against Grass types. Overall, Bite has a slight edge.

• Hydro Cannon is a fantastic choice for power and pairs best with Bite, while Ice Beam is perfect for counter-coverage and works just fine with either quick move.

• Hydro Pump hits hard and early in the animation, but Blastoise may not live to use it.

• Even when Hydro Pump is resisted, Flash Cannon is still worse.
• Skull Bash takes too long to access as a 1-bar move, but is harder to dodge than other moves. 
• Return is Blastoise' only three-bar move, making it attractive for defense. However, its power is low.

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.