I vote Giga Drain.
Meanwhile, here are some ideas for Gengar. (Uninspired to say the least but meh)
-Gengar is a powerful Pokémon who is also very frail. Aware of this, he must play very shifty, does his business by toying with the opponent’s condition and striking directly only when the time is right. Thanks to his ability, Levitate, his feet never actually touch the ground. For this reason, he may jump multiple times and has no need for a B^ to recover.
-B: Shadow Ball: Yeah we’ve all seen this b4 let’s move on.
-B>: Poison Jab: Gengar needs some recognition of his Poisonous typing right? Gengar lunges forward while his fist has a purple glow. The move works similarly to Ike’s B>. It sacrifices a lot of power, however, in exchange for the lack of charge. An opponent hit with the attack will become Poisoned. (A non-visible flower will be placed on their head.)
-Bv: Hypnosis/Dream Eater: Gengar’s eyes glow red as he emits a wave around him putting all hit by it to sleep. Any opponent caught sleeping near Gengar will automatically have their health sapped slowly, as you heal the same amount they lose. (I believe Leaf Barrett may know how to do this, Gardevoir has similar abilities.)
-B^: Confuse Ray: Confuse Ray works as a pseudo Counter. Gengar is briefly bathed in a light from above him. While under this light, Gengar is intangible. However, those foolish enough to attempt attacking Gengar at this time will become bathed in the light and become Confused (shield break of Deku Seed effect), giving Gengar the perfect opportunity to counter attack with Poison Jab, a fully charged Shadow Ball, or even unleash a Final Smash.
-Final Smash: Destiny Bond: Gengar’s entire body begins to glow. All of Gengar’s other attacks are disabled, but he is completely invinsible. Gengar must pursue his foes, setting them into a trap. The first opponent to make contact with Gengar (not an attack, but any contact at all) will be hit with at attack equivalent in strength to Marth’s Final Smash! This glow does not last forever though, so Gengar must play mind games with the enemy to lure them in. Confusing them or putting them to Sleep is a perfect plan.