Google today issue a recommendation for AdMob publishers to try the Adaptive Anchor Banner. Google says that adaptive anchor banners consider the device in use, the ad that a publisher is comfortable using, and the aspect ratios and performance of all available demand.
Smart banners, another option available in AdMob, only support fixed heights, so they often return creative that appears too small on high-res devices.
The standard sized banners return the same sized creative across every screen, which often results in ads that appear too small or too large.
AdMob’s Adaptive Anchor Banner is ideally sized across the user’s devices without the need for any custom code, and Google is recommendin to test adaptive banners against some of the existing banner ads to understand how they can help you maximize fill rates, engagement, and revenue.