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.

Max Battles

As a Dynamax Pokemon, Blastoise is a solid tank in the early metagame. Its high Defense stat allows it to make good use of Max Guard to tank and redirect boss aggro. Thanks to Water Gun's short 0.5s duration, Blastoise can charge up the Max Meter quickly in tougher battles. You don't really want Blastoise as your damage dealer, as Inteleon significantly outclasses it in that role.

With the release of Gigantamax Blastoise, regular Blastoise is outclassed except in situations where you may want access to Max Darkness (and even then, it's negligibly better). However, as Blastoise primarily wants to click Max Guard and Max Spirit to support its teammates, not having the extra power of G-Max Cannonade isn't a terribly huge loss. Nonetheless, it is still outclassed.

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.
• Rollout isn't great for Blastoise.
• Return offers nothing of any value.

Best PvE Defensive Moveset

• Rollout is the best way to go on defense.

• 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.

Rollout is preferable to Water Gun and Bite due to its high energy generation, fueling Blastoise's wonderful Charged Attacks.

Behold, one of the best charge moves in the game: Hydro Cannon (40 Energy, 2 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 70 energy cost it's quite slow and doesn't really complement Blastoise anyways.