TLDR; Gengar has great attack stats, and Shadow Ball does crazy damage. But his type advantage makes him pretty useless in this meta.
With the latest rebalance, Gengar is getting some attention.
- He has a base attack of 261, which is one of the highest in the game.
- Shadow Ball does a crazy 17.9 DoT (14.3 + stab), thanks in part to Hex’s high EPS
Sadly, Gengar has a lot of glaring flaws.
- His defense (156) and stamina (120) make him a glass cannon
- Ghost moves are not very effective against Normal. So much for countering Snorlax and Blissey.
- Ghost’s super effectiveness to Psychic is negated by Psychic’s super effectiveness to Poison.
Gengar’s role in this meta is highly situational at best.
- Sucker Punch / Sludge Bomb against Blissey’s Pound / Dazzling Gleam
- Hex / Shadow Ball against Donphan’s Tackle / Play Rough
- Hex / Shadow Ball against Heracross’ Struggle Bug / Megahorn
- Hex / Shadow Ball against Muk’s Poison Jab / Gunk Shot
Gengar has to skate around any defender moveset that contains Psychic and Ground.
Gengar tends to find a place against (non-Psychic) Fairy movesets of Normal types. Why? Poison resist Fairy. Also, Fairy is super effective against Fighting, which gives Gengar a brief chance to rank above Machamp.
The high attack stat is tempting, but I wouldn’t buddy or dust Gengar.
You’re better off investing in specialized attackers like Cloyster/Jolteon/Machamp. Or if it’s a hard-hitting generalist you’re after, Flareon with Fire Spin / Overheat is much more solid.