As an affiliate marketer, I would look at this in a different manner.
Not, "Which search engine generates the most traffic," but rather, "Which search engine generates the most traffic/revenue for ME?"
I find it much easier to get listed and ranked in Yahoo and MSN, so in my case, I would rank the search engines as:
Yahoo
MSN
Google
Since that's the order I receive traffic from and consequently make money from.
I'm talking about organic searches, of course, not PPC.