The question about the best Adsense unit is asked many times in webmaster forums each day and users will definitely get different answers depending on the person and his websites. Certain trends suggest that the medium and large rectangle are the best performing ad units when placed next to content while the leaderboard ad performs best in places where users would look for navigation. The problem I have with leaderboards is that they take away the most expensive ads if it is placed on top which means that the rectangle units near the content will earn less.

Generally speaking horizontal ads tend to perform better than vertically aligned ad units because horizontal ads follow the natural eye movement. The rectangles are actually that large that they can be seen as a mix of vertical and horizontal ad units.

Placing an ad unit is however not enough even if you have picked the best ad spot available. Ads should not stick out at all which means that position is one key element of a successful ad unit. The other equally important key element is the design of the ad unit. If you put borders around your ad unit and assign colors to it that are not used in your website it will automatically stick out and perform worse than ad units that integrate nicely into the site design.

Don’t make your ads look like ads because visitors tend to disregard ads if they stick out even if they contain interesting information and offers for the visitor

Some webmasters prefer the half-banner unit instead of the rectangle unit near their content. This has advantages and disadvantages. The advantage is that only one ad is displayed which means that it will be the highest paying ad which means that it will earn you more per click.

The advantage is the disadvantage as well. Only one ad means that there is no alternative if the visitor does not need the information of this single ad. Rectangles display four ads instead giving the visitor much more options. They will generate less per click usually but more in eCPM terms.

Another option that I have seen in various websites lately is to add a Adsense unit in the middle of the content. This can be done on websites with large articles. I suspect that those ad units, if implemented correctly, will generate a fantastic revenue for the webmaster.

Conclusion:

There is no way of telling which ad unit will perform best on your website. It will most likely be the large / medium rectangle unit and the leaderboard unit but if you do not test other ad units you never know for sure.

Success is determined by several factors including ad position, ad unit and ad unit layout. Other factors are also important including the type of site, the traffic sources or the visitors.