Most UFOs observed in daylight, when shapes and details can be seen, have been described as having simple geometric shapes: disks, spheres, cylinders and more recently, triangles.
Disk shaped airplanes have been flown, but none is known to have exceeded 150 mph, nor to have other capabilities displayed by UFOs. Difficulties in stability and control have so far prevented any disk shaped aircraft from getting beyond the stage of low performance prototypes.
Spherical aircraft have so far been limited to gas filled balloons, whose performance is at the bottom of the speed and maneuverability scales. Balloons can fly only as the wind blows and can be overtaken quickly by airplanes.
Cylindrical aircraft are unknown, as the lack of wings poses huge problems when it comes to such functions as taking off and flying level. Rockets and missiles are cylindrical and certainly are able to fly, but only as the result of great power in relation to their size. They can only fly upwards up at launch, and on a ballistic curve on their way to a target.
Triangle is the shape of delta winged airplanes, though the flight characteristics of triangular UFOs removes them from this category.
It is entirely possible that some radical military aircraft having one or more of these shapes are flying from super secret test facilities. But this would have to be a recent development unable to explain sightings of such craft during most of the past 50 years.