In my mind, Wikipedia. Why Wikipedia? Because it's the only one's who built its brand on search marketing (well, actually SEO, the non-paid equivalent). The rest did it other ways. The first four on the list are internet veterans, having been around since early days. Their brand building occurred well before search engine marketing came into effect. Now, obviously eBay buys a shitload of keywords, and Craig spams the search engines as well as anyone, but this referral traffic, while perhaps re-enforcing there brand, is really just an arbitrage play. The brands were built well before.
MySpace and Facebook, while they too spam the search engines, actually built their brand outside them. In Facebook's case, it was through tapping into universities and the accompanying viral phenomenon, and in MySpace's case, it was through good old fashioned email spam and the accompanying viral phenomenon.
As for CNN.com and ESPN.com, those are entirely different animals. Each of those has managed to pull off the difficult task of turning offline brands into effective on-line brands. This is something a lot of other companies have struggled with.
So that leaves Wikipedia, which, according to Hitwise, receives about 70% of its traffic from the search engines. And, to be honest, that actually sounds low to me....
So, I ask: How did Wikipedia manage to pull a brand out of its search engine spamming? Because Jimmy Wales is a personality? Because some sort of open source mojo? Uber-effective guerrilla PR? And why haven't other search engine spammers (like about.com) manage to pull off the same trick? Is it because they look like spam on the search engines, people think of them as spam and not a brand? In other words, is it possible that search engine marketing, while good for arbitraging, is actually harmful to brand building?