Gengar

Gengar makes a name for itself as a strong disruption pick in DOU. Mega Gengar is notable for being the better half of the two viable Shadow Tag Pokemon in the metagame and has a unique assortment of support and disruption moves such as Taunt and Will-O-Wisp that it can use in conjunction with its trapping ability and high Speed, which is enough to outrun the whole unboosted metagame except for Deoxys-A and Mega Aerodactyl; these qualities make it adept at picking off or neutering key threats for its teammates' benefit. Its regular forme is also viable, adept at using a nifty array of resistances and immunities to support teammates with Icy Wind and the aforementioned Taunt and Will-O-Wisp, making it a good check to notable threats such as Mega Kangaskhan and Landorus-T. Both formes are also no slouches offensively, with Mega Gengar even boasting a very high base 170 Special Attack, and their Ghost STAB makes them useful against most Trick Room setters. Both Gengar formes are also frail, however, making them unable to switch in on most hits effectively and being easily KOed by any strong or super effective attack; in particular, Gengar needs a Focus Sash to be actually able to take one hit minimum. Gengar's frailness also makes it easily picked off when at the wrong end of speed control.