Gengar

Under the full moon, Gengar enjoys mimicking the shadows of people, and laughs at their fright.  If you feel a sudden chill, they say Gengar is nearby- usually trying to lay a curse on you.
Gengar is the original Ghost-Type Pokemon.  Gengar is also part Poison-Type.  Like many other Ghost-Types, he doesn't have the biggest HP stat, and Gengar also has low Attack and Defense stats.  But who needs Defense- Gengar is a Ghost-Type, and he's impervious to Fighting-Type Moves, which is where many Physical Attacks come from.  Gengar also has a lot of Special Moves available as well, so he doesn't need the Attack stat either.  Gengar has average Sp. Defense stats and really high Speed and Sp. Attack stats.  The Speed part of Gengar is what makes him so useful; if you have low HP stats, you can go first before some damage is inflicted on you.
Gengar is best as a Specially-Based Pokemon.  He has moves like Shadow Ball, the sometimes useful Hyper Beam, Dark Pulse, Sludge Bomb, and even Perish Song by breeding.  Gengar can use the "Hypnosis-Dream Eater " strategy, and he can also use Nightmare, too.  The way I see it, Gengar is full of potential.
Gengar's only ability is Levitate.  You're immune to Ground-Type moves.  Without this ability, Gengar would have another Super-Effective Weakness.  Unless you're battling against an opponent with Mold Breaker, Levitate will always be handy.
So there's nothing bad to say about Gengar, and makes a great Ghost-Type for the main story, WiFi battles, first-time players, everything.  Yeah, Gengar's awesome.