CNET has an interesting article comparing the privacy policies of major search engines. According to that article Ask has the best privacy practices while Yahoo had the worst. One apparent weakness in the article was its focus on the companies' use of data (e.g., how long is it kept; do the companies rely on behavioral targeting of ads). While a company's use of data is clearly a major privacy concern, I would also be interested to see a survey which included information on how protective the companies were of the data that they do have. For example, Google actively fought the government when subpoenas were issued requesting information on searches performed by Google users (see this article (have to scroll down) for more information). To my mind, that should give the search engine giant a bump, especially given how eager most companies seem to be to hand over any and all customer information to the government (e.g., AT&T, whose behavior resulted in a lawsuit by the EFF as described here.
However, in spite of its inadequacies, the article still contained useful information. One interesting point is the discussion in the article of the effect of regulation on search engine privacy policies. For example, the article cited efforts by a group of European bureaucrats called the Article 29 Working Party as being a contributing factor in adoption of time limits for data retention by search engine providers. If there really is a causal connection, it would be a good example of how globalization can actually benefit consumer rights, since Americans would essentially be reaping the privacy benefits of regulatory pressures experienced by companies doing business in Europe.