Finishing the mission "Curtains Down" in Hitman: Blood Money with a good stealth rating.
The mission isn't too difficult, but it was hard enough that I couldn't just go in, guns blazing.
I killed the actor who played the Executioner, disguised myself as him, and then waited for the cue to kill the target. Once the deed was done, this started to play:
http://www.youtube.com/watch?v=FU3t5Xr-Tvc
Hell just broke loose. The second target, comes running over to the dead first target, and I killed him as he was running with a falling chandelier. This was one of the first times in the Hitman series that I moved as slowly as possible, just to take in the music.
This turned this game from a pretty good stealth game to THE stealth game.