If Aliens actually visited Earth, it would be VERY easy to prove without a doubt that they exist to the vast majority of people.
A combination of a few of these would satisfy me,
A peer-reviewed analysis of an unknown alloy or physical item that could only have been conceived by science and physics much more advanced than ours. One so advanced that it would require a new branch of physics or chemistry to even be explained in the first place.
Peer-reviewed scientific information regarding current phenomena that we are 50 years behind on.
Gigapixel photography of another planet, including some of species in a biopsy, including some tissue samples. All of which would be analyzed under peer review by unconnected scientific establishments around the world.
Other forms of data which couldn't be obtained with our current and even unknown technology here on Earth. Such as the entire contents of another planets equivalent to the internet being uploaded on our Earth servers for the entire public domain to observe, scrutinize and organize into a coherent system which we could learn from.
An autopsy of a deceased alien publicized fully, held in public for individual recordings, and containing doctors with great public records from all over the globe, also recorded and transmitted live in HD from multiple angles to remove the possibility of any editing.
Here's what doesn't do it for me,
People speaking.
Most of this crap would lead to new technology becoming available which could be used for warfare. If we are to remain the Earth Emperors we must harness and protect this type of information and never let it fall into the hands of commies, brownies, etc