I'd go with the Khmer Rouge. However, there are lots of ways to define "worst":

• The Khmer Rouge was simply genocidal, and they were killing loads while in power.
• The Japanese Communist Party is very ineffective. Barely anybody in Japan even considers them. While their revisionism makes them akin to a SocDem party of any other nation, the staggering part is their size, that being as big as an after-school club.
• The CPUSA became full of revisionism after WWII.

Does worst mean fell into internal revisionism the most, was least able to grow and spread thruout the people, or least able to survive the USA? I mean, where does one draw the line between a communist party that is incredibly revisionist (ie, the CPUSA), or a bourgeois party that brands itself left-wing (ie the Movement for a People's Party)?