As per my knowledge the maximum no of traffic came from Google. But if i have to keep Google out of the box then from my side its Yahoo for me........
Bcaz i have seen that maximum number of people are using yahoo as there mail as well as search engine (portal). Though bing is good one but it is not so much popular as yahoo and Google. So my vote goes for Google first and then to yahoo.
Its a nice question
Yahoo or bring both are useful to bring traffic for our site but serp's of target keywords should be very good. We always try to improve our site serp's on every search engines to get good traffic.