Gengar

Type Tier
Ghost / Poison OU
Statistics
Stat
HP
60
323
Atk
65
228
Def
60
218
SpA
130
358
SpD
75
248
Spe
110
318
Name Item

Starter Gengar

Leftovers
Moveset
~ Hypnosis
~ Explosion
~ Ice Punch / Fire Punch
~ Thunderbolt

This Gengar works quite well in the starting position. Electric + Ice is still a nice combination of attacks to have, Explosion keeps Blissey from just sitting her ass down, and Hypnosis is a great, perhaps surprise, move (though not as much anymore). Since Sleep Talk runs amok in GSC, it's best to have a sleep-inducing move on something that doesn't usually have one. Fire Punch is an option for Forretress/Steelix.

Name Item

Dynamicpunching Gengar

Leftovers
Moveset
~ Thunderbolt
~ Fire Punch
~ Dynamicpunch
~ Perish Song / Destiny Bond / Explosion

Thunderbolt and Fire Punch cover the Spikers like the set before, and Dynamicpunch is for the common Snorlax, Tyranitar, and Blissey switch-ins. I used Perish Song to get rid of minor annoyances like Cursing Sleep Talkers, and it's helpful when your opponent's last Pokémon is a Curselax or something like that. However, you can easily put Destiny Bond or Explosion there if you so choose. This set works fine, however the set before is recommended seeing as Dynamicpunch has terrible accuracy, and the Curselax will likely have Earthquake, which can take you out before you have a chance to let Perish Song run out.

Other Options

Gengar's movepool makes him difficult to handle at times. You might have a good idea of what someone is carrying, but you should always be on your toes when fighting this guy. Gengar has potential to be able to take out any of his counters. Alakazam, Misdreavus, Espeon, and Hypno take Shadow Ball hard. Quagsire, Rhydon, and Golem can get Giga Drained, Steelix could get Fire Punched, Houndoom, Snorlax, Umbreon, and Tyranitar could take a Dynamicpunch...and Raikou can get Exploded on. Gengar can also be a mean Perish Trapper, with his high Special Attack he can use various attacks such as Ice Punch and Thunderbolt to kill pseudo-hazers and pimp the show from there. Since Explosion is now the standard on Gengar, you can be sneaky and use Destiny Bond to snuff out anything that thinks it can kill Gengar without taking a lot of damage from Explosion—Curselax, Donphan, Tyranitar anyone? Gengar even learns Counter, which is awesome for bouncing back a non-STAB'd Earthquake from Snorlax or something for a KO.

Opinion

A very effective Pokémon when dealing with Spikers, and works wonders along with Spikes due to his blocking Rapid Spin. Gengar is so much fun to play with and so horrible to play against. Even though his Attack is low, it's good enough for one to seriously consider Dynamicpunch and Shadow Ball in addition to Explosion in his moveset. He's fast and unpredictable, always letting you have an extra edge. Gengar can Explode completely out of the blue and completely change battle momentum. A definite X-factor.

Counters

Blissey, Snorlax, Umbreon, Tyranitar, Raikou, Houndoom, and Alakazam are probably the most dependable counters, but Gengar can mess them up if he has the correct moves. Other counters include everything else listed in the Other Options section, but just remember to take it all with a grain of salt. He has the ability to take out any counter of his: he is hardly ever shut down.