Regirock

Regirock, similar to the other two legendary titans in Pokemon GO, is a defensive monster. In spite of its monolithic defense stat it cannot be placed in a gym. It has access to both a Rock-type fast move and charge move, which puts it well ahead of most Rock-type Pokemon in GO. That said, you're much better off just using Golem or Tyranitar.

Best PvE Offensive Moveset

• Rock Throw + Stone Edge is Regirock's only really viable attacking set in light of its poor Attack stat.
• Lock-On is for PvP only. 

The addition of Lock-On is a significant improvement for Regirock, featuring the best energy generation in the game with 5 EPT (Energy Per Turn). While its pitiful damage output of 1 DPT (Damage Per Turn) is often too low to farm down an opponent, the massive energy gains are needed for Regirock's expensive charge moves. 

Stone Edge (100 Power/55 Energy) is the easiest to use, and a powerful option for general use. The legacy move Earthquake provides Regirock with excellent coverage when combined with Stone Edge, and is more reliable than Focus Blast. Focus Blast (150 Power/75 Energy) is a genuinely powerful but highly expensive option. Given these moves, Regirock is best used as a closer, as it appreciates not having to worry about opponents shielding its slow and imposing charged moves.