Conspiracy theorists has been stuck with this issue. UFO is an abbreviated form for unidentified flying object. This issue with this flying saucers has been going around for decades and here in this list, it is all about how the conspiracy all started.
• AREA 51 - USA
For many of us, the beginning of believing in aliens started here. Many of the UFO sighting here had started here since 1950s. The theory here is that that UFOs were broken into pieces when it was crashed in this military base. But the news released by the government is that they were testing out new flights and weapons. The CIA has encouraged the rumors of UFO to spread around that so that Russian Govt wouldn't doubt them.
• Trans-en-Provence Case - France
In France a farmer in 1980s discovered two saucers shaped flying objects and also let a clear fine print of it. This case is one of the most famous for UFO sightings in France. This man has described the UFO in a very detailed manner telling us that it was like two saucers on top of each other and it was at a height of 1.5 meters and the color was rusty brown. But in the end the police looked at the evidence and mentioned that is looked like two cars that just parked but the eyewitness says otherwise.
•Frederick Valentich - Australia 
Frederick Valentich was an Australian pilot who served the Air Force who disappeared while flying a training aircraft. While he was in his last flight he saw a UFO which was above 300 meters from his aircraft and was also moving at very high speed. When his officials from the control room asked to describe it, he said that it looked metallic and had green light and was revolving around him. When the officials asked him whether it was an aircraft he replied that it didn't look like an aircraft. A static sound interrupted them and all contact between him and the officials were lost. It was somehow concluded by UFOist that he was abducted. But the Australian government said he was confused by his own reflection in the opposite aircraft while flying upside down.
• Shag Harbour - Canada 
This is an interesting case as we have multiple sightings of this incident. First the Canadian pilots saw the UFOs parallel to their aircrafts while flying. Next 2 children while sitting in their porch also se the exact same UFOs and the another navy officer also saw the UFOs while trying to locate. But in the end the aircrafts went missing. There are multiple witness yet there is no answer to this question by the Canadian government.
• Tehran UFO - Iran
Here an American fighter pilot in Iran claims have to malfunction when the UFO came near to the flight but these claims were immediately resolved as the flight usually has its own electrical and technical malfunctions and the these sources were immediately dropped down.
• Phoenix Lights - USA
The spacecraft here went to form a triangle in the sky and this was spotted by several people in the borders of Phoenix and Nevada. Another incident were the lights were present in a linear form in the sky. For the first incident it was claimed to be lights misdirected. But for the second incident not many reasons were given as many individuals took a live video as well as pictures but for this incident also it was shown that it was all light flares.
• Kaikoura Lights -
New Zealand
A cargo flight service saw the lights in the Kaikoura mountains of New Zealand here they followed them but the UFOs disappeared and then reappeared again. They saw white lights continuously. The objects also appeared on the radar of Wellington service. But the New Zealand authorities claims it to be some sort of lighting from squid boats which was reflected from the clouds.
Finally if you are interested in more of UFO sightings check out Praveen Mohan in YouTube as well as Facebook. He shows how ancient India might have alien and UFO sightings where it is inscribed in the temples of India.