The below table makes it very clear which ad format you should be using. The 336 x 280 large rectangle receives the highest click through rate by far. It is followed by the 300 x 250 small rectangle, and then the CTR really starts to drop off from there.

Average CTR by Ad Format
Format CTR
336 x 280 7.46%
300 x 250 5.58%
728 x 90 3.16%
120 x 240 2.99%
160 x 600 2.44%
120 x 600 1.33%
468 x 60 0.53%

The reason that these two formats do better than the others depends on which format you compare them to. The 468 x 60 banner clearly is the worst victim of banner blindness. Again, “banner blindness” is the fact that most Internet users today are so used to seeing advertisements that anything appearing in the shape of “tradditional” web advertising on a page is subconsciously ignored. The 728 x 90 suffers less from this phenomenon, but it suffers just the same.

The thin and tall formats (120 x 240, 160 x 600 and 120 x 600) get a lower CTR because they usually don’t fit well anywhere except on the right or left sides of a page. But the right and left sides of a web page are usually reserved for navigation and smaller advertisements. The real meat of any web page is in the center of the page. So while these formats are not bad in themselves, the typical positioning of them gives them a lower CTR.

You should be using the 336 x 280 large rectangle format, or if you don’t have room for that, use the 300 x 250.