if you are talking about Search engines Then google is best , But if you are talking about whole services from Both providers Then Both are Good , Google have gmail best in there way , yahoo have chat room and messenger , Google have blogspot , Yahoo have mybloglog , i know its new but very good for all kind of peoples like (
SEO, Business, Community ) And for making money Via Ads google is best for all , but yahoo publisher is just For USA , So google is again ahead from yahoo