The Wow! Signal
The Big Ear radio telescope at Ohio State University in the United States made the initial discovery of the Wow! Signal in 1977. A golf course has long since taken the place of the radio telescope (far from its history-making glory days).
The 72-second Wow! Signal was a strong narrowband signal that stood out against the noise around it like a lighthouse in the night. For forty years, the source was completely unknown, with the prevailing idea emerging in 2017 that the source was actually from a comet (a hypothesis that has been heavily scrutinized as of late).
The Wow! Signal is the sole radio broadcast that jumps out as possibly coming from an alien, in contrast to some of the other radio signals on this list. Jerry Ehrman, the person who examined the data report, noticed it and famously scrawled "Wow!" next to its uninteresting designation of 6EQUJ5.
Ehrman sat on the edge of his seat, hoping the signal would come back, but it never did. However, we could now have an answer courtesy of the Gaia telescope from the European Space Agency. Kind of. An amateur astronomer had the brilliant idea to search for sun-like stars in the area of the sky where the signal was first detected using data from the over 1.3 billion stars that Gaia has cataloged.
He found 67 stars that at least somewhat matched these requirements, and one of them was an exact match. Exoplanets in this region are currently being searched for. Ironically, the source of the signal is still unknown at least right now.
