Primarina

Primarina is the majestic Water starter of Gen 7, and a welcome addition to Pokemon Go. With Hydro Cannon it's a pretty solid Water Type, and its basic moveset also makes it a good Fairy Type attacker, but it's not really meta-defining by any metric.

Best PvE Offensive Moveset

• Waterfall and Hydro Cannon pair well together to fulfill a Water-type attacker role.
• Sparkling Aria is good, and the fact that it's 3-bar makes it situationally great, but it generally falls behind Hydro Cannon
• Hydro Pump falls behind.
• Charm and Moonblast are slightly weaker, but enable Primarina to be a decent Fairy-type attacker.
• Disarming Voice comes out a lot faster than Moonblast, but its damage output is significantly lower.
• Psychic has no use for offense. 

Best PvE Defensive Moveset

• Charm is recommended for defense due to ubiquitous use of Fighting-type attackers, as well as most more common usage of Grass and Dragon attackers compared to Poison Pokemon. 
• Waterfall isn't bad at all on defense, but is more likely to be resisted. 
• Psychic is better for defense against potential Grass/Poison attackers as well as Fighting-types. 
• Disarming Voice is good for defense, but using it alongside Charm allows Steel and Poison Types to hard-wall Primarina.
• Sparkling Aria is objectively great, sporting high power and great flexibility. Just don't pair it with Waterfall.
• Hydro Cannon is good, but on defense it's not as good as Sparkling Aria
• Hydro Pump and Moonblast take a long time before they can be used, but the former hits much faster. 

Both Fast Moves feature high STAB damage output with poor energy generation, with different typing coverage. Either can be viable depending on the rest of your team composition and PvP format.

Primarina's Community Day move, Hydro Cannon, is an incredible low cost move that helps make up for the poor energy generation from its Fast Attacks. Disarming Voice is another relatively cheap STAB option that will see more play than Primarina's more expensive Charged Attack options.

Moonblast is a great move, but Primarina struggles to find room for it as both Disarming Voice and Hydro Cannon provide it with much needed consistency. Psychic can provide coverage against Poison-type Pokemon, but it is rarely more useful than Hydro Cannon. Hydro Pump was formerly a staple on Primarina to threaten Fire- and Steel-type Pokemon, but it is now severely outclassed by the much more reliable Hydro Cannon. Sparkling Aria is a solid move with a moderately cheap cost, but is completely outclassed by Hydro Cannon and should only be used as a non-Elite move alternative.