According to this report, AOL returns the safest sites and Yahoo! returns the most "risky" sites. Some key points:

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* While all search engines return some unsafe results, listings at Yahoo! are particularly risky.

* Since the release of our search engine study in May 2006 ... Google, AOL, and Ask now return safer results, while Yahoo! and MSN return riskier results.

* MSN has the highest percentage of risky sponsored results (10.7%), while Ask's sponsored results are safest (6.5%).

* In our tests, Google's first sponsored result on the top of the page is somewhat more likely to be safe, relative to Google's positions 2 and 3. Among Google sponsored results listed on the right side of the page, the top two results are less risky than subsequent positions. We've noticed that many #1-rated Google ads are actually non-profits receiving free advertising through Google.org. The special characteristics of these non-profits probably explain the relative safety of Google's top-most position: These ads tend to promote sites that are safe (no spyware, scams, or the like), and these ads also tend to be top-ranked since their creators need not make cash payments to Google.

* Our tests suggest that adult search terms are twice as likely to lead to unsafe results as are non-adult search terms.

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The "Category Analysis" section (relating to Google Zeitgeist search terms) is quite interesting too.

Full report here:
http://www.siteadvisor.com/studies/search_safety_dec2006.html

Via: The Official AOL Search Blog
http://journals.aol.com/aolsearch/searchblog/entries/2006/12/12/search--safety/232