Use the 336×280 ads!
I have been playing for 4 years and generating A LOT of money from this Pay Per Click program make me realize that only 1 format in adsense that generates lot of click and higher CPM. The format is text format 336×280.
336 x 280 is good in CTR because characters and links inside, likely the same as common website/blog text and link inside the page, hence visitor thinks it is a part of the content.
I have tried some other format, and no one gained better CTR and higher CPM than 336×280 format, even the image version recent months has a lower CPM than format I said above.
If you play AdSense, I encourage you to try the most rewarding AdSense format, 336 x 280.