I preferred the Dark Knight's two-face... it never really seemed to be a guy who had developed a split personality. He just seemed like a guy who had lost everything, and was angry about it, and the coin thing was just sort of a method for him to feel in control of his rage - the whole point of his character arc was that batman, in his eyes, flipped a coin, and he "won", and the woman he'd grown to love "lost". So it's just a way of absolving him from the bad actions that his unbridled rage at his situation makes him want to do.
I think Harvey Dent always had that streak in him, he's just always had a reason to fight his battles a little less directly, using the law instead of a gun. But once the accident happens, he's lost that outlet. The Joker sits down with him and says "You can kill me if you want, but if you really think about it, the batman caused all of this, if he didn't exist, I wouldn't have tried to taunt him using your life, so if you're going to blame me, at least also blame the bat." And Dent's slightly warped view sees that that's the truth - he always didn't care for the batman. He used him as a tool, but he hated that he existed. Once the batman had the nerve to spare him instead of rachel, he decided, screw batman.
Of course, all of this isn't really hidden - the movie pretty much says all of this stuff directly, but it's a different take on Two-face compared to other incarnations of him. You never get a sense that the good Harvey Dent is actually still in there in the Dark Knight. At least I didn't. Harvey Dent IS Two-face, and Two-Face IS Harvey Dent, there's no line in their character in The Dark Knight.
Just going to also mention that I actually liked Tommy Lee Jones's Two-Face, although it was admittedly very very very silly, and didn't give the gravitas the role benefited from in Aaron Eckart's hands. I don't think Joel Schumaker or Tommy Lee Jones had ever actually read up on the character, because he had none of the story elements that make Two-face interesting. I like to look at the Schumaker films through the lens of the 60s tv show, just because it's the only way it makes sense. If you do that, they're not quite as bad.
As for The Riddler, I love the Riddler. I prefer him to Two-Face (and actually prefer him to The Joker). The Riddler has always been fairly consistent, although I do have to say one thing about him. I much prefer him in his three-piece suit. I'm sure we all do, but sometimes, they still put him in that giant onesie, that was never very good. In Batman Forever, they put him in it more then anything else, and Jim Carrey's package kept me from fully engrossing myself in that movie. His first scene as The Riddler where he goes to try and entice Two-face to his side, he was looking pretty awesome, then it got worse.
I find The Riddler's character is very interesting, but I doubt I can say more then these guys did. It's also interesting to note that the Riddler is one of the few villains that in many canons knows who Batman is, but he understands that it doesn't matter who batman is. He doesn't want to defeat Batman, or kill him (generally), he just wants to best him Mentally. His ultimate goal is to have Batman concede that he's won. That he can't figure it out. Everything else is secondary to that. Like the Joker, it's all a game to the Riddler. And he wants to beat Batman at that game. If he wanted to get rich, both the Joker and the Riddler are both smart enough to get as rich as they want, but what they really want is to beat the Batman. I just don't read "beat and kill" in the Riddler. He's slightly less psychotic then the Joker, who's ultimate victory would include Batman's death.