One of my favorite pokemon ever since the series was introduced. Gengar will undoubtedly have a lot of competition this generation for a spot on teams with the advent of newer ghosts (shanderaa especially) but one thing that sets him apart from his other ghost counterparts is his speed. Gengar has always been at the top of the speed tier among ghosts, and he continues to remain on top even after 4 new generations of ghosts.
Pokemon-Gengar
Item-Choice Scarf/Choice Specs/Black Sludge
EV's (252 SpA, 252 Spe, 4 Hp/Def)
Nature-Timid/Modest
Set "Suicide Trick Anti-Lead"
Moves:
-Trick
-Destiny Bond
-Shadow Ball
-Venom Shock/Thunderbolt/Sludge Bomb/Focus Blast/Energy Ball/HP Fire
Due to the prevalence of many other special sweepers, this set doesn't focus on sweeping but instead attempts to cripple any opponents (most notably leads) that are getting in your way. You can still cripple those troubling walls with trick and when you have no more use for gengar, you can take an opponent down with you who tries to finish you off (I find this especially useful against scizors who think they can come in and finish you off with bullet punch). Shadow ball is in the third moveslot for a solid STAB attack, and the final move depends on your team. Venom shock works especially well when combined with toxic spikes, so roserade or foretress with toxic spikes make excellent partners for him. Thunderbolt is always useful against water types, and with the prevalence of overconfidence gyarados, it just gives you even more reason to use it. Sludge bomb is another useful stab attack, and is your best STAB poison move if you're not usuing venom shock. Focus blast is useful for hitting those steel types that will attempt to wall you (especially doryuuzu). HP fire can be used if you're having issues with scizor (but only try to use this when he switches in, as CB bullet punch will kill you anyways), foretress, and natorrei (Focus blast will also hurt him quite a bit). Finally, energy ball is an option if you're having issues with swampert.
As far as items go, choice scarf is generally your best option, because if you mispredict your opponent it's better than giving them a free choice specs. If you're looking for raw power, choice specs is your best option. Finally, tricking a black sludge onto your opponent in conjunction with venom shock is also a decent option, but with the prevalence of steel types, choice items are usually a safer bet. It also helps gengar last a little bit longer (then again, with his pitiful defenses he probably won't be taking many hits anyways).