Adsense Formats
Everyone wants to know what the most successful Adsense ad formats are. By formats I am referring to the Ad block size or type. For example:
Large Rectangle
Banner
Skyscraper
Square
Half Banner
Wide Skyscraper
Horizontal Text Links
Vertical Text Links
Etc
Many polls have shown that the large rectangle is often the best performing advertisement size. Skyscraper, leaderboard, banner and medium rectangle are also popular choices.
However despite polls, it all comes down to your website. If a large rectangle doesn't fit will in your site design, then there is little point in using it. Only in some cases will this particular Adsense format work well.
It is vital that you test and test again all the possible ad format combinations on your site. I like to change my formats once per month or more, depending on traffic levels (the more traffic you get the quicker you can run tests).
After a while you will be able to gauge which ad blocks work best. Remember to use channels to track each ad! Only through channels will you know which ad block is being clicked on.
Google have provides what is know as a "heat map" which shows you potential hot spots for Adsense ad placement.
You can view it here:
https://www.google.com/adsense/support/bin/answer.py?answer=17954
As you can see on the map, the top and left parts of the page are the hot spots and it is here that you should be placing your ads to begin with, after which you should follow up with further tests and alternative placements to see which brings you the highest CTR.
This quote from Google sums up how you should think about Adsense placement:
"While this heat map is useful as a positioning guideline, we strongly recommend putting your users first when deciding on ad placement."
They are advising of course to keep your users in mind first and then deciding on where to place your ads.