Hello Jeremy and Team,
Benjamin Burns from (Constratus)here. I would say it deeply depends on the target market. I find that Google adwords sends the most traffic, MSN converts the best, and Yahoo is great for targeting the female market.
If I had to choose just one, I would go with MSN adcenter.
Top 3 reasons why MSN adcenter is my winner
1. Costs of keywords are still relatively cheap
2. Conversions are wonderful, get use to double digits
3. The Adlab tools they plan to launch are better than any other PPC engine to date.
The audience I’m sure is aware of this, yet the main stream public is so… Google centric that they are hypnotized in thinking Google is the only engine that delivers. Go Save money, convert in the double digits in some cases and test the tools in (adlab).
I have to hand it to MSN, they did a great job with their PPC platform, now if they can only spend time working on their search engine 
Happy success with Auction Ads!!
Cheers!
Benjamin
About the author...
Jeremy Schoemaker – who has written 2415 posts on ShoeMoney.com.
Hi I am Jeremy Schoemaker and ShoeMoney.com is my blog. 99% of the post here are done by me but you will see others occasionally make guest posts. This blog is fun to write but for my day job I run several online companies.
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I agree Benjamin. My first affiliate campaign ever was on MSN Adcenter. I spent $2.50 on clicks that day and made $25 in commission. Been doing it ever since.
I think most of the huge players like adcenter’s cost, but they can’t get the volume of clicks that they need.
Yes, the traffic numbers are tough to get at times, yet I’m a fan of launching everything under the sun. I once thought just roll with one or two offers that are working. If your not losing money I say launch as many offers as you can.
Cheers!
Benjamin