I have several I'm pretty proud of, but this is probably my favorite. I really like my players to come up with detailed backgrounds for their PC's. In a campaign I ran for nearly 10 years, one of my players had written the reason her ranger had left the "corps" was to flee an abusive relationship by a fellow ranger.
Now I try to make time "in game" coincide with "real time". So two years later, her ex-lover showed up. The ranger's player was very surprised by this. I ran him, as an NPC, for several adventures, and during the whole time, he tried to show how much he still loved her and how much he'd changed. She wanted nothing to do with him, and he eventually attacked and tried to rape her. She and the rest of the party chased him away, thinking that was the end of it.
Now, about 6 months further on, the party, while exploring the tunnels beneath an ancient and ruined city, are attacked by a pack of ghasts, led by a wight. Just as things begin to look their grimmest, her ex-lover charges in and dives into the fray. The last words they hear from him are, "Run! Get out while you can! I'll hold them off!" They get out of the sewers, and wait and wait, but he never appears.
2 months later, her character has gone off ahead of the rest of the party to scout the location where they are supposed to rendezvous, when she is attacked in the ruins of a wizard's tower. Fighting in the darkness, she can't make out her foe clearly. During a lull in the combat, the creature steps into a pool of moonlight, and in a harsh, gutteral voice says, "I swore I would always love you, and that we would always be together. If I can't have you in life, then we'll be together forever in death!" She gets a good look at the creature; her ex-lover, now a wight, who has stalked and tracked her since his fall to the undead monsters in the sewers.
The player actually screamed, and nearly burst into tears at the shock. And when I say player, I don't mean her character. She was so caught off guard by this that we had to stop the game so she could calm down.
Yeah... I'm VERY proud of that one.