In this story, Joker is most prominent in the flashbacks to Jason Todd's death. The Joker is forever embedded in Batman's mind as one of two reasons that Jason Todd is gone (the other being Batman himself). Mr. J is malicious and cruel... but all in the effort to creating just the right kind of situation that will make him laugh for hours on end. Like a baseball pitcher, he only wants to to setup a joke and let someone else deliver the punchline. In this case... the punchline was Jason Todd. Joker is not present during The Gotham Contract (he's out there somewhere in the world), but his etching in Bat-Family history is a huge part of Batman's journey throughout the story. His actions are horrendous, but they need to be acknowledged to move forward in the mission to protect Gotham.

I would say Jared Leto's Joker would be the aim here, but... I'll be reasonable. Take Leto's Joker and be a bit more creative with it. Give it a Hamill twist or a bit of Ledger... whatever you think best fits the character. Remember, the version of Joker portrayed in the Gotham Contract is a flashback to the night Jason Todd was killed. Therefore, he looks much different and therefore talks (and laughs) different. But you can determine how big those differences should be. Either portray Leto's Joker to a T or throw in quirks of another version (or your own version) to spice him up a bit. Whatever works for you.

In my mind, this Joker is clever, but doesn't always show it. His voice should scream creepy and energetic at the same time. It should undeniable that this guy is keeping in loads of anger and energy that could lash out whenever he feels things won't go his way.

Joker references:

https://youtu.be/3N9Iv-9hI4E?t=1m19s This scene in the club is full of good reference for his voice. Also good to see how he acts towards Harley and towards others.