I know that when Nickelodeon launched in 1979, Pinwheel was its defacto flagship show. As matter of fact, to give you a point of reference, by 1985, Nickelodeon devoted at least five hours to Pinwheel. You Can't Do That on Television appears to be the first true "mega-hit" show that Nickelodeon had. It was from roughly 1982-1986 (when Double Dare premiered), the highest rated show on Nick. Double Dare could be considered the first "modern" Nick show as it wasn't an foreign import (YCDTOT was a Canadian show) and it really cemented the channel's "messy, fun" image going forward.