Google for sure. It dominates the Internet. Yahoo is second but not a close second. Google have a lot of power on the Internet.
Google is the best, no doubt even if it shows fewer backlinks than yahoo. Google picks only few backlinks and displays them while google displays them all. If you are well ranked in google then you are automatically ranked on other search engines. So always focus on google for doing SEO.
Yahoo and Google are almost equal on early 2000 but when Google expand their products (like Google Earth, Android, Chrome, Google Plus, Blogspot), Yahoo left behind. I am using Yahoo because of their mail, nothing else.