Castform (Rainy)

Rainy Castform doesn't have the stats to be viable in the meta. At least it looks cute in your dex though. 

Best PvE Offensive Moveset

• Water Gun is the superior pick, partly due to STAB. 
• Tackle lacks STAB and can't be Super Effective against anything.
• Weather Ball Water is a strong move, and also more efficient as a 3-bar. 
• Hydro Pump falls a bit behind in performance, is okay if TMs are lacking. 
• Thunder doesn't fit Rainy Castform's role as a Water-type. 

Best PvE Defensive Moveset

• Water Gun has the edge on defense with STAB. 
• Tackle performs similarly, but doesn't have STAB and can't hit for Super Effective damage. However, it also leaves Castform a bit less walled out by anything that resists Water.
• Weather Ball Water as a 3-bar move can be used more often before Castform faints. 
• Hydro Pump hits the hardest and is the most difficult to dodge, but Castform might faint before it can use it. 
• Thunder is somewhat inferior, as it deals less damage and doesn't provide very meaningful coverage. 

Water Gun is preferred for its superior energy gain compared to Tackle, in addition to STAB.

Weather Ball is a strong move for its low energy cost, gets STAB, and can be used frequently. Thunder is recommended for additional coverage, though it's not exactly cheap on energy.