Haven't playtested this yet, but here is a start to the analysis.
Gengar
[SET]
name: Protect + 3 Attacks
move 1: Shadow Ball
move 2: Hidden Power Fire
move 3: Thunderbolt
move 4: Protect / Focus Blast
item: Life Orb / Ghost Gem
nature: Timid / Modest
evs: 4 HP / 252 SpA / 252 Spe
Why this set deserves to be on-site:
-Gengar is very powerful, and capable of taking on some of the top threats of the VGC metagame
-Shadow Ball hits just about anything hard, especially if boosted by Ghost Gem, things that are taken down include Chandelure and the Rotom formes
-HP Fire is for things such as Amoongus and Whimsicott who are very present in the VGC environment
-Thunderbolt is for coverage, and hits bulky water types that may show up, Focus Blast could be used here to nail Tyranitar, but other than that, its pretty much useless
-Choice of item is pretty much dependent on the team
Additional Comments:
-Choice items can definitely be used on the Gengar, but you would need to take off Protect and replace it with Focus Blast. Specs Gengar has a lot of damage potential, at the cost of being locked in
Teammates & Counters:
-Anything with powerful enough priority can take out Gengar
-Conkledur can do the job, but will take a lot from any attack if Gengar isn't Choiced
Gengar

[SET]
name: Protect + 3 Attacks
move 1: Shadow Ball
move 2: Hidden Power Fire
move 3: Thunderbolt
move 4: Protect / Focus Blast
item: Life Orb / Ghost Gem
nature: Timid / Modest
evs: 4 HP / 252 SpA / 252 Spe
Why this set deserves to be on-site:
-Gengar is very powerful, and capable of taking on some of the top threats of the VGC metagame
-Shadow Ball hits just about anything hard, especially if boosted by Ghost Gem, things that are taken down include Chandelure and the Rotom formes
-HP Fire is for things such as Amoongus and Whimsicott who are very present in the VGC environment
-Thunderbolt is for coverage, and hits bulky water types that may show up, Focus Blast could be used here to nail Tyranitar, but other than that, its pretty much useless
-Choice of item is pretty much dependent on the team
Additional Comments:
-Choice items can definitely be used on the Gengar, but you would need to take off Protect and replace it with Focus Blast. Specs Gengar has a lot of damage potential, at the cost of being locked in
Teammates & Counters:
-Anything with powerful enough priority can take out Gengar
-Conkledur can do the job, but will take a lot from any attack if Gengar isn't Choiced