The Phoenix Lights, Roswell and The Travis Walton incident. Those are my top three sightings for likelihood of being authentic.
The Phoenix lights were witnessed by thousands and filmed by press, citizens and military.

Roswell is pretty self explanatory.

The Travis Walton incident is one of the most scrutinized and documented abductions in history. Everyone involved has stuck to their story for decades and there has been no other plausible explanation for his injuries or disappearance.

I would have to be an absolute fool to think we are the only intelligent life in the universe. Given how old the Universe is, it only stands to make sense that someone figured out warp drive, or worm holes, long before we even considered theorizing with them. So why wouldn't they stop in to watch us blow ourselves up. I'm sure we make for good weekend TV.