actually, google gets the most traffic, but I have better results on Yahoo because yahoo appeals to a larger portion of our society than google does. in other words you get a larger variety of potential customers
If you don't want any risk then go with the public. Yes! Google is the best for PPC advertising services.
Google sometimes tends to close accounts without bothering of explaining any reasons (not obvious, by the way). Probably, there are a lot of sites which have problems with Google although their traffic does not brake any rules directly. Yahoo is #2 to try at least. But it's interesting if there is something to name #3 after all. I mean, if you failed to apply for both and your site is not adult or something...