Gengar's high Speed and Special Attack stat alongside its helpful Ghost-type STAB make it a good candidate for use in Ubers. Gengar has access to STAB Shadow Ball to hit the many Psychic- and Ghost-types of Ubers for super effective damage. It also gains very useful Normal- and Fighting-type immunities with its Ghost typing, the former allowing it to revenge kill Rayquaza, who would normally annihilate faster Pokemon with a boosted ExtremeSpeed. It has its downsides, however, as its paper-thin defenses will crumble under any attack that isn't Ground-, Normal-, Bug-, or Fighting-type. If you can see past its extreme frailty, though, Gengar can be a force to reckon with.
Gengar's high Speed and useful typing make it a great Choice Scarfer in Ubers because of its ability to check many common threats and its immunities to types like Fighting, Normal, and Ground. Hidden Power Ice is used to eliminate threats such as Rayquaza, Garchomp, and Shaymin-S. Shadow Ball is Gengar's main attacking move, as it hits Pokémon like Mewtwo, Latios, Latias, and Giratina super effectively and gains STAB. Focus Blast can hurt Darkrai and Lucario, both of which are outsped by Choice Scarf Gengar (unless Darkrai has a Choice Scarf of its own). Trick can be used as a last ditch effort, and only should be considered when Gengar is low on HP or if it is truly imperative to cripple an opposing wall, sweeper, or Wobbuffet.Team Options & Additional Comments >>>
This Gengar's function is twofold: it is a revenge killer who can switch moves, and it is a lure. Gengar can OHKO Rayquaza 100% of the time with Icy Wind, as long as Stealth Rock is down (about 80% of the time without). Icy Wind also has the beneficial side effect of lowering your opponent's Pokémon's Speed by 1 stage, which allows Gengar to outspeed all Pokémon who reach a Speed stat of 522 (including base 100s with a +Speed nature and a Choice Scarf). By using Hidden Power Fire, Gengar can eliminate Scizor, which is OHKOed by Hidden Power Fire even when the sun is not out; however, it is only a 2HKO in the rain. Metagross, on the other hand, is 2HKOed by a Hidden Power Fire with no weather (in the sun it has a small chance to OHKO, but it's only a 3HKO in the rain). Tyranitar is also handled relatively easily, since there's a 100% chance of an OHKO even when Tyranitar has maximum HP and Special Defense and a +SpD nature as long as there is Stealth Rock and a layer of Spikes up.Team Options & Additional Comments >>>
Thunder or Energy Ball can be used to hit Kyogre and/or Groudon harder. However, Latias or Palkia can generally deal with Kyogre and Groudon better, although it still can be handy if your team does not have one of these two. Explosion is another possibility on the Choice Scarf set, as it provides a fast way to almost instantly take out a Pokémon (remember to use a Hasty nature in this case). Destiny Bond is a possibility, as you can eliminate specific counters to your other Pokémon as well as threaten any would-be counters to Gengar. Finally, Substitute is a possibility on the Life Orb set to aid in prediction, but will mean Gengar loses a lot of HP from Substitute + Life Orb.
A Choice Specs set is also possible, as Gengar has very good Special Attack and Speed stats. However, Gengar loses its ability to revenge kill as effectively (which is really its niche), and the role of Choice Specs user is better filled by by Mewtwo, Kyogre, Palkia, or Dialga.
Checks and Counters
Pursuit users give Gengar a lot of problems, especially Choice Scarf users, as they can OHKO Gengar even if it doesn't switch out. The common Pursuit users in Ubers are Tyranitar, Scizor, and Metagross. Of these, Metagross is probably the most threatening, as Gengar can't OHKO it even with a predicted super effective attack. Specially defensive Jirachi can paralyze Gengar with Thunder Wave and wear it down with Iron Head while healing itself with Wish.
Kyogre can be a problem for Gengar, since it has high base HP and Special Defense stats. Without Explosion or Thunder, Gengar will not be able to deal with a Kyogre who has Calm Mind + Rest / Sleep Talk. Ho-Oh will take little damage from Gengar that lack Thunder or Explosion (note that Thunder only has 50% accuracy in the sun) and OHKO Gengar with Brave Bird or Sacred Fire.
Finally, Blissey will pose a large problem for Gengar unless it carries Explosion. Blissey is immune to Gengar's STAB Shadow Ball, and it also takes pitiful damage from its other moves. Also, even when Gengar uses Explosion, it can be predicted with a switch to a Ghost-type. This will cause Gengar to basically kill itself while doing absolutely no damage.