There is currently an extremely credible government whistle-blower who claims that the U.S has captured non human space craft.
https://www.youtube.com/watch?v=7_B1WdWhFrQ
If you want to hear a possible debunk. I hope the guy doesn't sound too condescending (he made many videos about it, so maybe he is getting tired of it or something), but I think he's pretty smart.
There’s a few things that are really rock solid: UFOs are here
I can't help myself from thinking about the "Extraordinary claims require extraordinary evidence" quote.
Also, the "UFOs" term is weird in the sense that some people use it to describe shit we don't recognize in the sky and other people use it to refer to an alien spaceship/drone whatever. I'm guessing you mean the latter since, for me, it's logic that they are many occasion that we don't understand/recognize something from afar.
Personally, I don't think we are visited by aliens. I wouldn't be surprised if there's life out there somewhere, since the universe is huge in a ridiculous way, but life could range from bacteria or whatever, to animal, to "super smart" being that aren't able to coexist and live on a planet full or resource while being on the verge of bankruptcy.