I think Harvey works best when he is about the same age as Bruce. I like Lee Pace as an actor, but I don't think he can pull off the wholesome, noble aspect of Harvey. He always kinda comes off as a dick to me.
My first choice is still Oscar Isaac. He just exudes the sort of charisma that Harvey should have, but boy can he flip and go dark. I think he could really pull off the Harvey/Two-Face dynamic. He deserves a chance to go to town on a solid mainstream role after being wasted in X-Men and Star Wars.