Didn't Yahoo stop there ad network in 2011? And suggested Chitika? So correct me if i'm wrong but than it would be between Chitika and Google Adsense, still Google Adsense is the best ad network on the market followed by Infolinks.
Google is Best for PPC. Bing is Good for PPC. Conversions/signup's from Google and Bing may vary in a broader way. Bing helps the business in gettin leads and Google lead your business towards the increase in revenue.
Now days Google is best online service provider. Following service make a different place for Google. Adsense, webmaster tool, analytics, adword, Most imp it's search engine
Are u asking as a publisher or as an advertiser? As a publisher, Google has yet to see tough competitor. Media.net by yahoo-bing is new and looks good. As an advertiser, consider Facebook too..