Cofagrigus

Cofagrigus is a genuinely great Pokemon... in design only. Sadly, our sarcophagus friend lacks the moves and power to have any real impact on the PVE meta as a whole. It's a cool defender that can use its bulk and medium CP to work well for mid-term gym defense strategies, but that's about it. This coffin is just too easy to bury.

Best PvE Offensive Moveset

• Shadow Claw should go with Shadow Ball for Ghost Type damage.
• Astonish is a downgrade. 
• Zen Headbutt can work with Psychic if you're desperate for a Psychic Type attacker.
• Dark Pulse is an inferior attack, having nearly identical coverage to Shadow Ball, but lower power and no STAB.

Best PvE Defensive Moveset

• Zen Headbutt sports the highest base power, but lacks STAB, and won't be Super Effective against most Ghost-type attackers. It also leaves Cofagrigus extremely vulnerable to Dark Types.
• Shadow Claw is better than Astonish. It still leaves Cofagrigus vulnerable to Dark Types, but to a lesser degree .
• Shadow Ball is the move of choice, hitting hard with STAB
• Psychic is great, and can put a huge dent in Machamp, but it's generally weaker than Shadow Ball, is more frequently resisted, and lacks STAB.
• Dark Pulse is a step down in terms of damage output, but with nearly identical coverage.

The addition of Shadow Claw is a much needed improvement for Cofagrigus, and is significantly better than other Fast Move options. 

Shadow Ball is great on just about any Pokemon, especially with STAB. Dark Pulse could be chosen for its slightly faster accessibility since it costs 5 less energy, considering Cofagrigus has no cheap bait moves to pressure shields with. Psychic doesn't provide good value for Cofagrigus when factoring in its moderate energy cost.