Originally posted by Cogito
Joker, if he wants to.
I'd imagine he plays to many games and has to much varied motivation for his chaos/mayhem. Jason is simple and a pure force of killing intent regardless of victim, surroundings and care for seemingly anything including his own well being. It's why I imagine characters who are capable of brutality and non hesitant murder like black Adam don't really fit the mold and fully qualify as answers to the question. Even they (joker, black Adam) could find themselves in situations where they will hesitate due to various reasons where Jason would hack away with no care.
Not to say Jason doesn't either. as the only thing that I Can remember made him stop was his mother (Freddy appearing as such) but to answer the question as to Whois 'more' a killing machine I'd still go with doomsday who would run over his mother (if he had one) and leave a fine mist in his wake.