If you are looking for Best adsense format (i.e. the one which brings highest revenue) then it is 336×280. It has been reported repeatedly as well as acknowledged by most of the webmasters that the big ad unit brings maximum clicks and revenue as it is very much on the face.

Infact here are the Top 3 best Adsense Formats in terms of revenue -

1. 336 x 280 Large Rectangle (300×250 can also be put in the same category) - The ads of large rectangle takes lot of space and are probably the most visible among all ad formats. They are used at the top or bottom of most of the pages and because people read in F format, they get maximum exposure.

2. 728 x 90 LeaderBoard - Leaderboard were heavily used in the past and were the most popular adsense ad unit but nowdays with more sleak designs, they have been replaced with 46×060 ads. Normally leaderboards are used at the top for maximum exposure.

3. 160 x 600 Sky scrapper is the third most popular and high paying adsense format. it is used in the sidebars, mostly on right. Its great for the sites which have lot of sidebar space as you can always put these ads without compromising much on user experience or on content. You can see the image here

Most of the adsense ads format are anyway variants of these three, personally I prefers 468×60 as seen on this blog as they are less intrusive and fit well in content.

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