Many publishers repoted that 336×280 large rectangle, 300×250 medium rectangle and 160×600 wide skyscraper performs well.
For some reason 336 X 280 did not work well in my sites ( works well in entertainment related sites). 300×250 medium rectangle worked well in all the sites. 160×600 wide skyscraper blends well, but CTR( click through rate) is not that great compared to 300×250 Medium Rectangle.
Checkout Google AdSense Online Banner Ad Sizes, Ad Formats. Ad unites perform different way in different sites, but some are definitely more efficient than others. The key is to keep trying different ad types until you find that works best for your website/blog.
Share your experience in the comments section.