In my opinion, the most watched content on YouTube is content that contains vlog and social experiments. Because nowadays there are so many famous youtuber originating from works by creating their YouTube content by creating their daily vlogs. Not only that, he also usually collaborates with several other popular YouTube users so that the content is more widely known.
Apart from downloading vlogs, one of the most popular content on YouTube is social experiments. Social experiments is an experiment carried out by a youtuber in a public place whether intentional or unintentional, usually the youtuber performs an experiment that is like working on someone else intentionally in a public place.
In my opinion, these are the two most popular content in YouTube content, because nowadays there are a lot of famous youtuber, which of them are great and reliable vloggers.
If you mean speacifically the video with more views it is “See You Again” (Wiz Khalifa, feat. Charlie Puth). I'm guessing the popularity of the song was due to its relation to Fast and Furiois and Paul Walker's tragic death. The video passed Korean surprise, Psy, with his Gagnam Style, which had about a billion hits, with almost 3 billion views now.
Music videos seem to be the most viewed materials on Youtube.