To me the most compelling evidence is always that from Air Force pilots and NASA scientists who with all of their considerable knowledge of all things air and space are unable to identify or explain things they have seen, or have come to the conclusion even after they have considered all possibilities that what they have seen was not based on human technology. A fair few (and some of notable rank) have come out and spoken up about this now and while they of course have no hard evidence, their testimonies are very compelling.
I always remember in the mid 90's being a passenger in a car where the driver had his own Cessna and we were going to fly it.
We were travelling down the A50 to Uttoxeter when I noticed an object to my left and I jokingly said 'A UFO'.
We were travelling for ages with this very strange object over in the distance and my mate decided to pull over into a layby and another car pulled behind us because they had been watching the same thing.
All 5 of us were standing there amazed and my mate flew light aircraft and couldn't make out what it was.
About 1 minute later the object turned and it was a bloody helicopter.
To this day we don't know what position it was in but IT WAS LIKE SOMETHING WE HAD NEVER SEEN BEFORE.
If it had flown behind a mountain while still in a weird position to us, I would be telling a UFO sighting story now.
In the late 70s I saw a neighbour looking up at the sky and within minutes the whole Grove was out.
One neighbour called the Police and two were standing with us watching this UFO hovering in the sky somewhere over Stoke.
My next door neighbour came out, asked what we were looking at, looked up to the sky and said 'Harrier Jump Jet' and walked back in.
A few minutes later it took off and it was clearly a Harrier Jet that had decided to hover.
IT WAS LIKE SOMETHING I HAD NEVER SEEN BEFORE.