Greninja

Greninja is the Kalos Water-type starter of Gen 6, and one of the most popular Pokemon throughout all generations. Upon its introduction to Pokemon Go however, Greninja falls behind various other Water-types who have superior movesets and/or stats. However, with the introduction of Water Shuriken and Hydro Cannon into its movepool, Greninja actually becomes a potent Water attacker. Though it still trails behind Kyogre and some Shadow Water-type Pokemon, Greninja can be an amazing budget option. Watch out for its frailty, though.

Although it is not introduced into the game yet, Greninja has an alternate forme called Ash-Greninja, which sports a much higher Attack stat. While we don't know how (or even if) it will be implemented, one thing is for sure - its raw DPS is so ridiculous that even Primal Kyogre is a fair bit behind it. That said, it's still just about as fragile as regular Greninja.

Best PvE Offensive Moveset

• Water Shuriken, available starting from August 12 2023, is Greninja's best Water-type Fast Attack. Bubble is a decent move, but is inferior on Greninja.
• Feint Attack is slightly better on offense, but can only pair with Night Slash.
• Hydro Cannon* is Greninja's best Charged Attack by far.
• Surf is a good move, but is a significant step down from Hydro Cannon.
• Hydro Pump is a step down, but still decent if TMs are lacking. 
• Aerial Ace has no use. 

Best PvE Defensive Moveset

• Bubble is generally better for defense, with slightly better damage and energy gains.
• Water Shuriken is slightly weaker than Bubble.
• Feint Attack is falls behind stat-wise, but isn't resisted by Grass or Dragon-type attackers. 
• Night Slash can be used frequently as a 3-bar move, gets STAB, and is overall quite decent.
• Aerial Ace lacks STAB and is a bit easier to dodge, but covers against Fighting, Grass, and Bug-types. 
• Surf benefits from STAB, and is decent as a 2-bar move. 
• Hydro Pump likely won't be used before Greninja faints, but is the hardest to dodge and also the most damaging. 

Water Shuriken boasts by far the greatest energy generation and is Greninja's preferred Fast Attack. Bubble deals very slightly more damage for considerably less energy gain, making it undesirable. Feint Attack is mediocre all around despite providing extra Dark-type damage.

Night Slash costs the least amount of energy, benefits from STAB, can be used frequently, and has a small chance to proc its self-buff. Hydro Cannon is an excellent STAB move and can be fired rapidly thanks to Water Shuriken's insane energy generation. Surf is a good move, but is strictly inferior to Hydro Cannon as it deals less damage for the same energy cost. Aerial Ace is quite underwhelming; its coverage against Greninja's weaknesses is likely not enough given its poor damage and greater energy cost. Hydro Pump demands too much energy to use.