Two-Face finally overcoming his reliance upon his coin by letting his old friend Batman go, even though it came up scarred (which he declared would mean the inmates could kill him), without letting either Batman or the other inmates know it did. The little smile he gives afterward will probably give you one too.
Also the very end of the novel. Dent looks at the Tarot deck which, despite the psychiatrist's good intentions in weaning him onto it to give him a broader range of choices, had psychologically paralyzed him to the point where he wet himself because it took too long to decide whether to go to the bathroom. He says, "Who cares for you? You're nothing but a pack of cards," and knocks them onto the floor, indicating that he's finally able to make a decision entirely on his own.
The original script states: "Two-Face smiles a smile intended only for himself, slipping the coin into his breast pocket. At the end, he has transcended destiny and made himself free, if only for this one time of his life." The 15th anniversary annotation by Morrison further notes: "Harvey Dent transcends his role here and steps out of the game, ending it with an act of defiant compassion."
YMMV may vary on both of those. Consider Joker's line about the outside world being the asylum, and insanity being a safe place to hide. Consider Harvey's belief that God made the world because he made a scarred-side-up coin flip, and therefore all of existence is evil and painful. With both those things in mind, Harvey was probably playing the coin-toss straight, since ceasing to exist, or being stuck permanently in an over-run Arkham and being slowly driven insane, would mean a cessation of pain and misery, whereas leaving would mean having to repeat the thankless cycle of going back to Gotham and feeding more criminals back into Arkham only for them to escape again (an act which is, in story, considered corruptive and evil by some). This troper would list this as an Alternative Character Interpretation, but she thought that was honestly how it was supposed to be read.
Or maybe it's both. Remember, Harvey was once a lawyer. And what do lawyers do? Twist around the laws to get the result they want. And in the end, EVERY decision has its good and evil consequences, so one could make an argument for ANY action being good or evil. In other words, maybe Harvey, in his small moment of compassion, CONVINCED HIMSELF that letting Batman go was the evil option...
Alternatively Harvey's choice is supposed to lead to a Fridge BrilliantCrowning Moment of Heartwarming: when God flipped the moon and it fell scar-side-up, He still made the world because some things are too important to leave to chance.