"Ingenious" can mean almost anything to almost anybody, so it's not surprising that this thread has degenerated into people just naming their favorite shows. It's hard to say Wheel of Fortune was ingenious when Down You Go came before it. It's hard to say the fill-in-the-blank Match Game was ingenious if...well...the 60s Match Game came before it.
Still, some of my choices:
Groundbreaking: Winner Take All
Yes, somebody would have come up with the idea of beating your opponent to the buzzer and the concept of the defending champion eventually, but for one show to have introduced both game show staples is nothing short of remarkable.
Scoring: Tattletales
Once they got rid of the "quickies", the simple elegance of the scoring system for Tattletales always impressed me, and the idea that the celebrities were playing for the studio audience was a masterstroke.
Game Play: Go
OK, technically, the Chain Reaction bonus round. Or as I like to think of it, Bob Stewart's last really good idea. Just when you thought he'd squeezed every last drop out of the communication games, he comes up with this. Twenty-five years later, I show old episodes to my jaded high school students and they're hooked. (And yes, I realize the structure of the show was flawed, but the idea behind the game was solid.)
Again, everybody's mileage will differ. If someone in this thread can say that a final 25-point bonus question was "rather clever", then just about anything's fair game.