If you're wondering which AdSense advertising formats perform best and which one you should use on your website, look no further than Google for the answer.
Google suggests that wider ad formats tend to outperform their taller counterparts. But why? Taller formats have a more reader-friendly format, as readers (your website visitors) absorb information in what are referred to as "thought units" or several words at a time. Publishing wider format lets consumers read more text at a glance without needing to skip a line and return to the left margin every few words as they would need to do with a narrower ad.
Google has found the most effective formats are the 336x280 Large Rectangle, the 300x250 Medium Rectangle, and the 160x600 Wide Skyscraper. Keep in mind that while these ad formats typically perform well, you should use the format that best complements your pages.