Google...because if Google goes down the crapper, that means the crap has really hit the fan. And Google's demise will take thousands of websites with it, who rely on adwords, adsense, and getting found in Google search results.
http://news.ycombinator.com/item?id=328227
Then again, I have until Oct. 2010 for that to play out (and I do have money behind my mouth on that one). 2009 is going to be the year for testing Google and its advertising income. They may gain market share from the little guys getting weeded out, but I'm still keenly interested to see how their income and EPS plays out.
If they're still holding strong into late '09 and pushing $5 EPS per quarter, well I'll be a tad bit poorer and a little bit smarter.
But I think Google will grow weaker everyday because of competition (But that can take more than 20 years).
IMHO the fact that Yahoo had missed so bad on web ads is one of the biggest failures in business history.