Banner ads offer the most screen real estate and a compelling ad design will deliver superior results.
Banner ads also offer the most options: four sizes (728x90, 300x250, 160x600 and 125x125), several ad formats (animated GIF, flash, video, etc.), and you determine how to target your audience (run of site or by geo region). Rich media ads (that include audio, video, expandables, etc.) and targeted ads incur an additional CPM charge.