Battle Moveset

A Good Moveset for Gengar would have to be this:

Attacks:

Focus Punch
Thunderbolt or Fire Punch or Psychic
Substitute
Ice Punch or Hidden Power *Ice or Grass* or Sludge Bomb or Shadow Ball or Giga Drain

Items Attached:

Leftovers

Preferred Nature:

Lonely (+Attack, -Defence) or Hasty (+Speed, -Defence) or Mild (+Special Attack, -Defence)

Strategy Using Gengar

This set (generically known as "McIceGar") has become the standard on the metagame for it's ability to destroy the Skarm/Forry/Weez-Bliss combo used by many stall teams. Although it's probably the best Gengar set for countering the commonly used Pokémon, when facing non-standard/AI teams it requires a lot of reworking.
Standard (non-ice Beam) Blissey can't touch Gengar except for inflicting Thunder Wave on it (and with a Substitute up that has no effect).
Skarmory won't want to face it since it is 1-2KOed by Thunderbolt. Weezing suffers a similar situation from Psychic and Forretress gets destroyed by Fire Punch (go with Thunderbolt all the time unless you know your opponents team is my suggestion).
Ice Punch (or Hidden Power *Ice*) take out Salamences and Dragonites as well as adding a psuedo-"Bolt-Beam" when used with Thunderbolt.
I've added Shadow Ball as an option since it 1-2KOs all the majority of Psychic Pokémon and 1-2KOs all the Ghost Pokémon (except Dusclops), most of which would cause some problems from their free switch in while you Focus Punch.
Sludge Bomb is added in there for some-what covering Celebi and other Grass types while making good use of STAB and those Attack EVs. It also gives you a move to use versus Ice Beam Blisseys
Giga Drain or HP Grass is there for hurting Swamperts (who will take all of Gengar's other attacks with ease).
Substitute can be dropped on this set for:
Will-o-Wisp [great for luring Blisseys into play] Hypnosis [Immobilises a Pokémon if it hits, but it's risky with the accuracy.Also lures Blisseys into play] Destiny Bond [It's a KO if it works.] Taunt [screws up any Skarmorys trying to WW/Roar you away as well as screwing up Celebi's Recovers] Thief [removes your own Leftovers and steal Blissey's]
or any of the last slot moves since Ice Beam Blisseys make Sub-Punching redundant. However, the majority of metagame Blisseys carry Thunder Wave. It's up to you really. Personally, if you choose to replace Substitute, I'd suggest giving it a Lum Berry, just in case it gets Thunder Waved or Wisped and you have no cleric (Heal Beller or Aromatherapist) available.
Also, as an added bonus, this Pokémon makes a nice switch in on Rapid Spinners and aids in keeping the Spikes alive (although Dusclops does a better job of blocking Rapid Spin and picking Genagr solely for being able to block Spikes is pointless, it's simply a nice luxury).
It's also one of those Pokémon that has no problems getting into a battle. With an immunity to Normal, Fighting and Ground manoeuvres and a 4x resistance to Bug and Poison attacks, it makes a good switch in to the majority of Choice Banders and slower physical sweepers. After that you usually have a free turn to set up a Substitute or use an offensive move on a Pokémon you expect to be switched in.
There are other useful move sets available to Gengar (and his huge move pool). For getting through the Battle Tower (R/S) an all-out-special sweeper (or one accompanied with a status attack) would probably be your best bet. For that I suggest:
Thunderbolt / Psychic or Fire Punch / Ice Punch or Giga Drain / Will-o-Wisp or Hypnosis
It can also make good use of Will-o-Wisp with such a high speed stat, immobilising plenty of physical sweepers. It can make a somewhat decent Choice-Bander, and it's one of the better all out Special Sweepers.
If you've claused Skarmory and Blissey (which plenty of people do) you'd be better off changing up Sub-Punch (and the EVs) for moves better suited to the role that you need to be filled within your team. With such a wide move pool Gengar can fill almost any role short of tanking/sponging.

EV Corner:

For the SubPuncher set:
Giving it 300 Speed is pretty standard (since it outruns Base 100 Speed Pokémon with max attack on a neutral nature). It wouldn't hurt to give it 301 Speed (or a little more) to outrun other "McIceGar"s.
Max Attack EVs is preferable for making use of Focus Punch. Without Lonely Nature, Focus Punch will still do around abouts 60% to Blissey, but having that extra attack is just handy for causing extra damage and making Sludge Bomb put more of a hurting on Celebi and making Shadow Ball 1KO Ghosts (and Psychics) more often.
Whatever stats you have leftover can go into Special Attack.
If you're obsessive you can make the HP divisible by 16 to work better with Leftovers, but that is really just a luxury you could do without. Leftovers only purpose is to replenish lost HP from Substitute. It isn't about to take many (Sub-less) hits.
A preferable Max IV distribution would be: {Mild or Lonely} 252 Atk / 180 Speed / 78 Satk.
With Mild it leaves you with: HP/261 Atk/229 Def/140 Speed/301 SAtk/346 SDef/186
With Lonely it leaves you with: HP/261 Atk/251 Def/140 Speed/301 SAtk/315 SDef/186
These would need to be readjusted for different Hidden Power IV distributions and for non-metagame teams, but the important stats are Attack, Speed and Special Attack which should be the main focus of your EV distributing.

Strategy Against Gengar

The great thing about Gengar is it wreaks havoc on the majority of overused Pokémon. The best way around Gengar is prediction, but the trouble with it is it has so many potential movesets that you can never be 100% sure that your countering it.
For the standard McIceGar set:
- Rock Blast is a nice counter since it breaks Sub then beats away at its weak Defence, however using it from a Rhydon or Golem is risky since if it carries a Grass move, you're doomed.
- Using Shadow Ball as it switches in will probably knock it out in one hit, same for the majority of strong physical attacks.
- Blissey's with Ice Beam or Pokémon with high Special Defence and a resistance to Focus Punch also cause the set some problems.
Things to beware of:
- Its access to status attacks (namely Will-o-Wisp and Hypnosis). It can immobilise the majority of Physical Sweepers with Wisp and completely immobilises a Pokémon with Hypnosis.
- It's ability to catch you by surprise with Destiny Bond before it faints.
- Access to moves that screw up stat boosters, namely Taunt (but also Haze, although that is a very unlikely possibility).
- And, like all Ghost Pokémon, it has access to all those horrible Dark TM moves that can mess around with your strategies (Thief especially can mess with your tanks, especially during a Tyranitar's Sandstorm).

Contest Moveset

A good Contest Moveset for Gengar would have to be this for the Smart Contest best with Calm, Gentle, Careful, Sassy Nature:

Attacks:

Mean look
Destiny Bond
Shadow punch
Confuse ray

Items Attached:

Green Scarf

Strategy Using Gengar

If you do the attacks in this following order you should have very few problems; 1st: Shadow punch, 2nd: Mean look, 3rd: Shadow punch, 4th: Mean look, 5th: Destiny bond.

Locations in Games

Ruby/Sapphire/Emerald Trade from FR/LG Colosseum Trade from FR/LG Fire Red/Leaf Green Evolve Haunter

Animé Appearences

Gengar has had a few Animé Appearences. First Ash saw one in The Lavender Pokemon Tower, after that a Giant one battled against a giant Alakazam by the ruins of Pokémopolis. After that Drake of the Orange League had one and used it in battle against Ash. After that Morty of Ecruteak Gym had one and used it in his battle against Ash Episode 001: Pokémon! I Choose You! Episode 023: The Tower Of Terror! Episode 074: The Ancient Puzzle of Pokémopolis! Episode 113: Hello Pumello! Episode 114: Enter the Dragonite! Episode 123: Illussion Confusion! Episode 183: A Ghost of a Chance! Episode 184: From Ghost to Ghost! Episode 229: For Ho-oh The Bells Toll!