Google beats all search engines http://googlefight.com/index.php?lang=en_GB&word1=google&word2=search+engines there is a google fight for ya!
When I think of search I think of Google. The best and the worst friend there is. Yahoo was the no.1 according to Alexa for a long time. It's over with that though. The big G has the no.1 spot now. I seriously doubt that Yahoo will ever be able to compete with Google from now on.
I've found, particularly recently that it depends what you're looking for. If you're searching for a biiig presence on the web, google will find it and rank it high but if you're doing a more detailed search, I've found that Yahoo and live.com are better. Google depends too much on pagerank, ie popularity - the big (popular) sites drown out the little sites which can often have the info you're seeking.
all my sites are indexed in both search engines and on average they appear in the same page of the results for any particular keyword, however google sends in like 10 times more traffic than yahoo, and I also noticed that people coming in from google buy more when compared to visitors coming in from yahoo (percentage-wise)
* Google sends by FAR way more traffic than any other search engine to my sites. * I prefer Google or Scroogle when doing searches. Your question is not very clear as to what you'd like to learn from it.