Unfortunately this is not an easy question to answer. The best approach is to try a few and and make your own choice. Not all search engines work in the same way. So if you were looking for references to acid rain some search engines might treat this query as acid and rain and others might treat the query as acid or rain . In practice this may not make much difference because most search engines take other factors into account, such as frequency of search terms, but it is worth bearing in mind that you will get different results using different engines.
Most search engines have a help page.
If you are interested in finding out the differences between the major search engines then try Search Engine Showdown for expert advice. Lancaster University approved page maintained by Michael Dunne .