Shadow Registeel

Shadow Registeel is a marked improvement over its base forme... but it still has to compete with monsters like Metagross and Shadow Metagross, so it just can't quite find a niche of its own.

Best PvE Offensive Moveset

• Metal Claw is vastly superior to Rock Smash. 
• Lock-On is for PvP only. 
• All three of Registeel's 1-bar charge moves(Hyper Beam, Flash Cannon, and Focus Blast) perform within 0.2 DPS of each other despite STAB or lack there-of, though Flash Cannon pulls ahead for sharing a type with Metal Claw.
• Zap Cannon* is strong, but consider actual Electric-type Pokemon instead.

The addition of Lock-On is a significant improvement for Registeel. This move features the best energy generation in the game while its pitiful damage output is often too low to farm down an opponent. Still, the massive energy gains are needed for Registeel's expensive charge moves. 

Focus Blast provides valuable Fighting-type coverage and has high damage per energy spent, but it's also a very expensive attack to charge up and use, and a single shield threatens to severely cripple Registeel's power potential. Zap Cannon is limited to specific events, but its incredible damage threat and debuff utility helps flip various matchups in surprising ways. Despite recommending two of the most expensive Charge Moves in the game, Registeel's long list of resistances and high bulk gives it the time it needs to severely pressure shields. Flash Cannon  is an attack with okay power per energy and it also benefits from STAB, but its poor coverage doesn't do Registeel any real favors.