i use gengar. he deserves the spot as # 1.
i use a different MS than most though.
Gengar @ wide lens
252 SpA, 252 Spd 4 HP
Hypnosis
HP Ice
Thunderbolt
Focus Blast
it works extremely well since everyone expects shadow ball, so i always save one move for surprise kills. eg, ill thunderbolt and focus blast, they assume that im not carrying HP ice but shadow ball, they send out any dragon (not kingdra), and then said dragon dies from HP ice. its really good. ive gotten 3 kills in 3 turns once since their start was salamence, easy OHKO with HP ice, then they switch to lucario, who was OHKOed by
focus blast, then they assume no shadow ball and bring in gyarados who got died from thunder bolt. it is fun. plus hypnosis and wide lens allows him to take on blissey. FB is a reliable 4hko(lol). it works though.