what is a viral video? A viral video is a video clip that gains widespread popularity through the process of Internet sharing, typically through email or Instant messaging, blogs and other media sharing websites. Viral videos are often humorous in nature and include televised comedy sketches such as Saturday Night Live's Lazy Sunday and Dick in a Box; amateur video clips like Star Wars Kid, the Numa Numa videos, The Dancing Cadet, The Evolution of Dance, the "Benny Lava" video; and web-only productions such as I Got a Crush... on Obama. Some "eyewitness" events have also been caught on video and have "gone viral," including the Battle at Kruger.
A viral marketing company, The Viral Factory, has put together a list of top 10 most viral videos that have made office life across the world finally bearable. Most viral video of all time
No. 1 online video has been watched more than 900 million times
check out All viral video collection from youtube
1. Star Wars Kid (900 million)

2. Numa Numa

3. Kylie Minogue: Agent Provocateur (360 million)

4. Exploding whale

5. John West Salmon Bear Fight