Mega Gengar

Mega Gengar is an absolutely tremendous Pokemon. With a massive attack stat and some of the nicest moves in all of Pokemon Go, it's one of the must-have Mega Evolutions without a doubt. Though it still suffers from the same psychic/ghost weaknesses that its base forme famously fights with, its sheer power is still more than enough to warrant a place on any team where it does Super Effective damage, and possibly even some where it doesn't

Mega Gengar is a safe and very effective investment. Period.

Best PvE Offensive Moveset

• Shadow Claw is a strong attack with decent energy gain. 
• Lick* is also a strong attack with less power, but faster energy gain than Shadow Claw. Both perform similar DPS.
• Hex and Sucker Punch are quite close in performance; the main difference is that Hex is resisted by fewer types.
• Shadow Ball is Mega Gengar's best charge move, sharing the same type as its best fast moves.
• Sludge Bomb also has STAB, but its Poison typing lacks synergy with Ghost-type attacks. It should be paired with Lick due to its superior energy generation.
• Focus Blast has niche uses vs. Normal-type foes, but overall is an inferior option.
• For Mega Gengar's legacy moves, Sludge Wave* has the same problem as Sludge Bomb's lack of type synergy in addition to being a 1 bar move that doesn't complement Gengar's frailty.
• Dark Pulse* is generally awful and offers almost no advantages over Shadow Ball.
• Psychic* is a good charged move, but it has no utility on Gengar.
• Shadow Punch*, Mega Gengar's Community Day move, is too weak to be useful on offense. Its only niche is being able to throw out a parting shot if Mega Gengar is about to faint and can't quite use Shadow Ball.