netmeg - 6:11 pm on Sep 7, 2010 (gmt 0)
A *smart* advertiser uses all the available ones. But not everyone is smart. The most popular seem to be the 728x90, the 160x600 and either the 300x or 338x squares. I personally make use of the 468x60 a lot, but there don't seem to be as many different ads in those spots.