I recently read that the adsense ads that are rectangluar seem to be most popular because they are flexible and can be used inline with the content - above the fold - or in the body, sides etc. 250 x 250 is one example. I use 160 x 600 for skyscrapers on my blogs.
For display ads, 728 x 90, 468 x 60, 300 x 250 and 120 x 600 are the most common. Standard popunder size is 720 x 300. Some other common formats are 160 x 600 wide skyscrapers and 125 x 125 buttons, often seen on blogs. For more, check out the IAB's ad format guidelines.