So far the best candidate for a a SETI signal was the Wow signal in 1977.

It met all the criteria and protocols except it was only heard once. So by the rules, it is considered “inconclusive”.

The Wiki article is authoritative and detailed, so I won’t be redundant.

http://en.wikipedia.org/wiki/Wow!_signal

Incidentally, the frequency is the wavelength (1420 MHz = 21 cm) of monatomic hydrogen. Some authorities feel that is a natural wavelength for a SETI beacon, because it is a natural emission that would be used by astronomers anywhere to map the galaxy’s spiral arms. I disagree, I suspect that there is so much noise from natural interstellar hydrogen at that wavelength that a more reasonable beacon would be at some harmonic of that freq, say 42 or 11.5 cm. But who knows?

Although the wavelength corresponds to a natural process, the narrow bandwith suggests this was an artificially generated signal. If it is indeed an ET artifact, it was probably accidental, a reflection or a random antenna pointing that illuminated the earth for a few minutes, and will never find us again.

The locational uncertainty of the signal’s point of origin is roughly the size of our full moon. I’ve studied detailed charts and photographs of that area, but there is nothing of interest there except a few faint field stars.