" Is there a game out there with the perfect dialogue tree construct? "Yep, Heavy Rain.
There's just 2-4 leaves to any given dialogue tree and unlike Mass Effect, or any game as far as I know, you have a limited time to choose your words. This creates a natural flow to conversations and a sense of risk to each choice you make.
For an interactive drama, or whatever Heavy Rain is, I sure played a bigger role in it than any so called role playing game.