Evian Babies is the most-viewed online ad, as recognised by the Guinness Book of Records, with 45 million views to date

"Evian's 'Roller-skating Babies' viral ad, created by BETC Euro RSCG, has been recognised by the Guinness Book of Records as the most viewed online advertisement to date.
Adding up views for various versions of the ad across video sharing websites, the Guinness Book of Records has bestowed the honour on the ad for having 45,166,109 views as of 9 November 2009.
The ad ties in with Evian's "live young" strapline and shows babies performing a series of increasingly skilled roller-skate stunts, set to the tune of 'Rapper's Delight'."
[Comment: Up to a point. It depends what you classify as an ad. If you include film trailers then Twilight is easily the bibbest, as detailed by this analysis by Visible Measures. However it is very possible that Evian Babies is genuinely the most popular TV ad that has been posted to a video site]
Source: BrandRepublic, 16th November 2009