No doubt, ET is out there. But as far as visiting little old Earth, I doubt it. Relativity makes it hard for far-away aliens to come around. Alpha Centauri, our closest stellar neighbor, is 4.35 light-years away. Even if aliens were circling that star, a round trip visit to Earth at close to the speed of light would take 9 years. I wouldn't want to take such a trip. It is unlikely that travel at near the speed of light is practical. What would be the energy source? What if something goes wrong in those 9 years? How do you decelerate if you lose power? A million questions...
But, you say, ET is a thousand times smarter than us, and millions of years more advanced. Perhaps. But I don't see why visiting ET's would try so hard to keep their presence secret. And if they're visiting so frequently as some would have us believe, where is the hard evidence?
There probably isn't any advanced civilization anywhere close, anyway. We would have probably detected their transmissions already. We are basically isolated by relativity.
As far as UFOs go, they do exist, but the acronym UFO means Unidentified Flying Object not Alien Spacecraft, simply Unidentified. What is a UFO to me could be an IFO to you. For instance, if a government is conducting secret aircraft test flights, one of those aircraft may look so unfamiliar to an untrained observer that he thinks it's an alien spacecraft.
Alien abductions? Give me a break! In my opinion, most of these claims are outright lies. Maybe some who claim to have been abducted by aliens do believe it, but I certainly don't. Our mind is a very rich and fertile ground for fantasies if we are not careful.
There has never been an iota of concrete, solid, scientifically testable proof of alien visitations.
There have been many incidents in which UFOs have been alleged to be present. Perhaps the most famous one was the Roswell UFO incident. The government says it was a top-secret research balloon. And you know what? I think it was. I've read many things about this incident and I haven't found any credible proof that it was anything other than that.
To me, the Tunguska Event is far more interesting. This was a massive explosion in 1908 in Siberia. It occurred a few kilometers above the surface of the Earth, it left no crater and was equivalent to about a 10–15 megaton blast, or about 1000 times as powerful as the bomb dropped on Hiroshima. The scientific explanation of an exploding asteroid does not, of course, satisfy UFOlogists. They claim it was caused by the explosion of an alien nuclear-powered spacecraft.
ET might come one day, but let's not rush it. We don't know if they will be friendly or not. Right here on earth we've had not-very-friendly humans try to subdue other humans so what can expect of an alien race who considers itself far superior to us?
Perhaps we're lucky in that interstellar distances are so vast...