Gengar

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 Extreme Speed. 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.