To think that Earth is the only planet with intelligent life in the whole universe, is rather unintelligent.
My personal opinion on any alien manned UFO's visited Earth, I think is very unlikely, based on the distances involved.
Has an unmanned UFO visited Earth, I think is possible, but would be of no consequence to the aliens who sent it here.
I 'm being a little narrow minded here, considering that possible tech that could be created that would not require any travel by anything to observe.
I remember reading about a process that involved a sub atomic particle, that would respond instantly to an identical particle simply being observed, regardless of were the particles existed.
Disregard any and all of the terms I used above, I don't remember the article in detail.
Maybe some body else here far more learned than I, will know what I'm rambling about.
Even Star Trek had to dream up a means of just communicating over the fast distances of space, I think they used Sub Space to communicate.