The Zipper!
Also, not sure if they technically still count as a "flats" or whether they should be considered "darks" due to their unconventional add-ons, but the now defunct Dante's Inferno, Abominable Snowman, and Dungeon at Miracle Strip in Panama City Beach, FL. These were a Chance Trabant, a Scrambler, and a Tilt a Whirl built inside domes, which included music and a light show during the ride. The Snowman in particular was the best Scrambler ever: the inside of the dome had MIRRORS, and half way through the ride a strobe light would kick in. Great take on an otherwise very standard ride. The Dante/Trabant was also very disorienting. At the moment it changed directions, and at the moment it first rose into the air, the lights would go out.