I voted other, simply because I don't believe the answer to be clear cut as just one of those options.
The term 'UFO' can apply to all of the above options.
And as for my own personal opinion, I also believe that more than one of those options is genuinely to blame for the phenomena. It is not one or the other.
I believe a small proportion are genuine extra-terrestrial craft (either manned/unmanned), some are experimental aircraft based on extra-terrestrial tech and scientific knowledge/advancements that aren't public knowledge. The other theory is that the tech may be terrestrial but not based on the present. Time travelling earth settlers whether human or another race. I always find it interesting how UFO's are often present surrounding key military sites where nuclear armament is present, and at places of both recent natural disasters and other key moments of time, and sometimes even just beforehand.
We all know that natural phenomena, flaws in perception, whether digital or natural can be to blame for some sightings. Fraud and deception unfortunately make up for a lot of so called sightings. I believe our own world governments may play on our willingness to accept things at face value. However it doesn't distract me from the fact that I genuinely believe we have had cases of extra-terrestrial visitation. Just what the percentage is out of all the cases, I wouldn't like to put a number on it.
I've been busy and haven't been on in awhile, but this is an interesting subject. I ran across a list of all reported sightings back to, like, the 1930's, and noticed an interesting pattern. Every year sightings increased exponentially right before the summer solstice. And I do mean EXPONENTIALLY. There would be, say, 10-15 sightings a month all year, then jump to 50, 60, 80, and more, monthly sightings as the solstice approached. Of course, these numbers climbed as the years went on, up to around the 1990's. The numbers grew to just under a hundred or so a month, to a thousand or more around solstice. This was a worldwide list btw. I don't really know what to make of it, and I imply nothing, but the pattern was definitely there. I'm going to have to see if I can find that list again.
That said....I agree with you. Some of them, I think, are not necessarily extra-terrestrial but maybe extra-dimensional. Very possibly terrestrial, but not from this frequency. Where you say "time-travelling" I propose "frequency-hopping". There are alot of things about "the creation" that are not widely known, but are completely experience-able by our bio-machine (they're a marvelous bit of tech, our bodies). But, I think a good portion of them, especially from about the mid-1950's on, are definitely our technology or, at least stuff that exists here and now. I also think that some of them are not technology at all, but consist of pure energy, no physical material at all. But, that's a whole different beastie that maybe deserves it's own thread.
Anyways, I could talk for days about this stuff, but I'm gonna knock it off now. Infinitely interesting subject matter.
-G