Gengar

Gengar has the honor of having access to literally all of the best Ghost type moves in the game. While Gengar's sky-high DPS has been surpassed by the even more extreme glass cannon Deoxys-A, it still tops the DPS charts and has fantastic overall performance for a glass cannon. Its Ghost typing hits a wide variety of targets for neutral damage, and handy resistances can make up for its extreme frailty. Unfortunately, Gengar is weak to the Psychic-types that it's supposed to counter, causing it to underperform in scenarios where total damage output matters more than DPS. Normal-types such as Blissey and Snorlax, common sights in gyms, also doubly resist Gengar's optimal movesets.

Gengar can earn its spot on raid teams against specific bosses to help beat the clock, but it requires immaculate dodging to be effective. An evasive Gengar will swiftly deplete large chunks of a raid boss’s HP. It's also worth raising at least one for the sake of Mega Gengar, which takes everything good about Gengar and makes it even better.

Dynamax

Gengar is also the first Ghost Type Dynamax Pokemon, and it can definitely put in numbers. Its relatively low TDO can be a bit of a liability in this game mode, especially when it's hunting down its fellow Ghosts and taking Ghost moves as well. Still, it's unlikely that we'll see much big competition for Dynamax Gengar in the near future... except for Gigantamax Gengar, so be ready for that.

...which got released merely a little over a week after Dynamax Gengar's release, so regular Dynamax Gengar's just plain outclassed now. However, Dynamax Gengar remains a strong option for those unable to get their hands on a Gigantamax Gengar.

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 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.
• Focus Blast has niche uses vs. Normal-type foes, but overall is an inferior option.
• For 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, Gengar's Community Day move, is too weak to be useful on offense.

Best PvE Defensive Moveset

• Defending with a Gengar is not advisable due to its very low bulk. That said, Hex hits the hardest and is therefore optimal.
• Shadow Claw has less power and less energy gain than Hex. Don't bother with Sucker Punch or Lick*

• Sludge Bomb and Dark Pulse both have 2 bars and are difficult to dodge; the former is benefited by STAB.
• Shadow Ball is powerful and narrowly resisted, but can be seen coming from a mile away.
• Focus Blast and Sludge Wave are unusable; Gengar will not survive to use them. 
• Psychic* has very limited utility.
• Shadow Punch is great on defense thanks to its low energy cost, STAB, and narrowly avoided typing.

Shadow Claw is the superior fast move in both damage output and energy generation. Don't use any other fast move.

Shadow Punch is a charge move limited to Community Day or Elite TMs, but is essential for baiting shields due its low 35 energy cost. Shadow Ball is Gengar's most efficient charge move in terms of Damage Per Energy (DPE), has incredibly high damage output with STAB and Gengar's high ATK stat, and hits most Pokemon for neutral damage. Focus Blast can be chosen as an option to fight against Dark and Normal-type Pokemon meant to counter Gengar, but its high energy cost makes it difficult to access. Sludge Bomb is generally not recommended, but can still be useful in certain formats to threaten Fairy-type Pokemon.