My short list of UFO designs I've become aware of in research on the subject.
Man-made
ARV/Flux Liner/Nazi Bell - man-made/unconventional technology.
@Gambeir provides much detail to it's theoretical design and function in this thread. Quartz-Crystal materials and multi-rotating center column in opposing directions.
TR-3A/B Triangular Craft - man-made. Allegedly a mercury type plasma spinning within a toroidal/torus shaped container.
OTC-X1 - man-made.
Tesla inspired saucer shaped. Either confiscated by MIB/or Otis T. Carr got cold feet when his scam was about to be exposed. Piloting involves consciousness interface with some type of crystal - according to a technician who claims to have worked with Carr.
Project 1794 saucer shaped hovercraft- man-made/jet propulsion. According to Dark Journalist this name is a play on "1947" or year of the Roswell event. This was a project to create a conventional technology but saucer looking craft as a way to make public think these possibly were the "flying saucers" seen. These hover-craft barely left the ground and performed nothing like "ufos" seen by public, similar only in shape.
If Flying Saucers Aren't Real, Why Did the Military Frantically Try to Build One
Extraterrestrial origin
Classic flying saucer/Sport Model - Theorized by ufologists as Tall Grey ET scout ship with small android Greys as pilots. Allegedly worked on by Bob Lazar (at least) using a stable form of Element 115. Matter-Anti-Matter annihilation and gravity magnification to warp space/time around vehicle and space travel. Could not be reverse engineered.
Symbiotic Alien UFOs - nuclear fusion containment engine powered. Large in size engine, indicating possibly a mothership or main ship. Semi-biological and consciousness interfaced as described by David "Rocketman" Adair, and recently
supported by insider Emory Smith's work on biological specimens. For the latter, the only explanation I can think how he was exposed to so many various specimens - was because they were mostly man-made genetically engineered in labs. That and combination with a brought down/shot down or acquired mothership that already had collection of many types of aliens/ETs on board. This would have been a massive haul for compartmentalized black project extraterrestrial specimen study within deep underground military bases. They recruited Adair to help reverse engineer the nuclear fusion container part, but when he asked where the beings who flew this ship were, they got angry.
Divine Celestial mus (ships of light) - Powered by a "Mu disc" monopolar system with 12 magnets on each disc and an over-unity power unit. Step-pyramid design. Piloted by human form ETs who originally seeded Earth hundreds of thousands of years ago, according to witnesses such as Norman Paulsen. According to him the real "Guardians of the Galaxy", not a movie.
"..we can attain light speed while hovering. This is accomplished by increasing the velocity of rotation within our propulsion drive units. As the drive units accelerate, we begin to traverse the radiant energy spectrum, moving from the infrared to ultraviolet. The entire mass of this vehicle is converted back into radiant life-energy particles as it traverses the spectrum. An outside observer on the physical plane sees the ship's image change color from bright red through orange and into gold; then bright green, followed by blue and violet. At this point we vanish from your so-called third dimension in a flash of light as brilliant as any sun."
Orbs - disembodied spirits that collapse into energy points in order to traverse multi-universes - guessing.