"The Most Interesting Man in the World" was a character created as a sort of mascot for the Dos Equis brand of beer. He has since become a meme in his own right - and now it has been revealed that he appeared in Star Trek as well. What an effective ad campaign: the Dos Equis spokesman is literally everywhere, and so is his association with the brand.
While I don't know the specifics of how much Dos Equis pays for advertising and how much revenue they receive in return, there can be no doubt that "The Most Interesting Man in the World" is a great advertising campaign. This is because it is funny and engaging, unlike many other advertisements.
While "The Most Interesting Man in the World" surely has garnered much attention in the past years and is definitely quotable and memorable, I think that calling it the most effective advertising campaign of the millennium is a bit much. I, for one, think it is a fun campaign, but it doesn't really inspire me to purchase Dos Equis. It just makes me laugh. Since I work in marketing, I know that the end goal of a campaign is to bring in customers and get some ROI, and this campaign, while recognizable and well done, doesn't make me want to buy the beer any more than I would without it.
While the Dos Equis marketing campaign has had an impact on pop culture, it pales in effectiveness when compared to such marketing juggernauts as Red Bull's 'Find Your Wings' or Dove's 'Real Beauty' campaigns. While Dos Equis has seen steady growth in sales since the launch of this campaign, it is mostly recognizable as an Internet meme, not as a means of selling beer. While it is one of the most recognizable advertising campaign of the last decade, it is not the most 'effective' in the sense of selling a product.