Rotom Mow

Rotom-Mow's moves were added to the Game Master long before Leaf Storm was implemented. Though the Pokemon was released a fair bit after Leaf Storm was added into the game, Niantic unfortunately never updated Rotom-Mow's movepool. Not that it would have made much of a difference; Rotom-Mow and its other formes will continue to be underwhelming in PvE.

Best PvE Offensive Moveset

• Thunder Shock and Thunderbolt make for Rotom Mow's only genuinely viable attacking set
• Thunder, Astonish, and Ominous Wind are all outclassed

Best PvE Defensive Moveset

• Astonish beats out Thunder Shock on defense due to its higher base power and great neutral coverage
• Thunderbolt is a 2-bar move with a fairly early damage window and is preferred to expand Astonish's coverage
• Ominous Wind comes out quickly, but it is weak and doesn't offer additional coverage
• Thunder is simply too slow

Thunder Shock sports very fast energy generation, though its damage output is poor despite gaining STAB. Astonish is a very bad move. 

Thunderbolt and Thunder perform similarly, but the former is recommended due to its lesser energy cost. Ominous Wind is Rotom's cheapest move and adds some coverage and a chance to boost Rotom's stats, but the move itself is very weak.