Google Zeitgeist lists the top search trends in 2012, but YouTube is always listed separately. The tabulation is much easier. Unlike search, the same videos aren't the most popular videos by the numbers year after year after year. The raw numbers are the trend, and YouTube doesn't hide them, unlike Google's search tools where the actual raw data is sometimes hidden for business purposes.
So, for 2012, the worldwide most popular YouTube videos are:
PSY's hit Gangnam Style reached the top of the list, and it became a record breaker. The Korean pop video became the first video to ever reach one billion views in the history of YouTube. The strange imaginary horse dance featured in the video and catchy tune prompted countless parody videos, as did Carly Rae Jepson's flirty Call Me Maybe. A cover video itself is what made Goyte's song Somebody that I Used to Know a huge hit. The cover band, Walk off the Earth, featured five people playing one guitar.
The previous top slot belonged to a Justin Bieber video, Baby. Justin Bieber (with help from Selena Gomez and some friends) was one of the first to record a cover version of Call Me Maybe, propelling the Carly Rae Jepson version to one of the top spots for the year. If you want to see pure awesome, though, look at the Sesame Street parody, Share It Maybe.