What was ABC’s first No. 1 primetime show?

When ABC launched its TV network in 1948, it was trying to break into a market dominated by CBS and NBC. The scrappy upstart tried westerns, crime shows, and even made a deal with Walt Disney that helped finance Disneyland in exchange for a weekly TV show. But despite all these efforts, ABC never managed to land the ultimate prize in television. What was ABC's first show to actually snag the number one spot in television ratings?