In my case, I had difficulty choosing between two. The first being the rather awesome James Bond RPG - I ran the "For Your Eyes Only" scenario three or four times for different people and had very different outcomes each time. Fantastic game.
The other was The Judas Grail scenario for the legendary RPG "Kult". Never finished running the game, the players kept getting distracted and going off in different directions, but it was great fun and Kult is still an amazing game that has stayed in mind all this time.