• R/B
• G/S
• R/S
• D/P
• Strategy
• Moves

Gengar

Sleep the first Pokémon, switch out, and save Gengar for later purposes as his Normal immunity comes in handy quite often. When your opponent least expects it, Explode for pretty good damage to anything that doesn't resist it. Mega Drain 2HKOs Golem and Rhydon, and when they are not around, Thunderbolt for a decent chunk of damage to everything else.

Other Options

Confuse Ray can be used to prevent the opponent from attacking if you get lucky—using it against a paralyzed Pokémon can increase that luck in your favor. Psychic is really only useful for hitting other Gengar, but is still an option. Seismic Toss is a decent choice for consistent damage to Pokémon that would otherwise take very little from Gengar's attacks, such as Exeggutor.

Opinion

Gengar is very useful in many respects. He is the fastest sleeper in the game, has a Normal immunity and very good Special, and can Explode when the timing is right, making him very difficult to deal with. While Gengar is the fastest sleeper in the game, Hypnosis will miss almost half the time. However, despite this shortcoming, Gengar is still a very good lead to Sleep the opposing Pokémon. His Normal immunity comes in handy quite often as it's the only surefire way to stop Wrap and can cause the opponent to lose a Pokémon if you predict his or her Explosion/SelfDestruct correctly. Gengar also packs the second fastest (behind Electrode) Explosion in the game, thus allowing him to put a stop to any threats causing you problems before they get out of hand. Overall, he is a pretty good choice despite his weakness to Psychic and Earthquake, because he is difficult to predict and can screw up quite a few common Pokémon in the metagame.

Counters

Alakazam is the best counter—he's faster and Psychic will do 63-75% damage. Rhydon will OHKO Gengar with Earthquake, and Golem will most of the time—they need to watch out for Mega Drain, though. Starmie will do a decent amount of damage with Psychic, but Psychic isn't used very often on Starmie, and Gengar's Thunderbolt will hurt it very much. Hypno and Exeggutor both do a ton of damage with Psychic, but won't OHKO. Tauros ties Gengar in Speed and will 2HKO him with Earthquake. The best way to beat Gengar is to hit him with a strong Earthquake or Psychic as they will usually 2HKO Gengar.