Mark Hamill as The Joker and Solomon Grundy. Definitely. That'd suit my own personal inner dialog (yes dialog, not monologue, there's too many damned sides to my thoughts) so I'd need him doing two of his iconic voices.
Grundy: "You hurt Stabnex? Stabnex hurt YOU!!"
Joker: "Now why did you have to say that? Now I have to kill your family and make you beg for more."
Grundy: "You hurt Stabnex? Stabnex hurt YOU!!"
Joker: "Now why did you have to say that? Now I have to kill your family and make you beg for more."