How Google determines order of Adsense ads on a webpage

Google Adsense supports suggests that the ‘main’ ad unit must appear first in the HTML code. By saying ‘main’ ad unit - most probably Google is referring to the ad unit with the highest CTR.

Read this new adsense support query How do I define the first ad unit when using multiple ad units?

The multiple ad units system serves ads to each ad unit in order of their appearance in your HTML code. In most cases, this will be the same order as the ad units appear on your page. However, if your code uses DIV tags or other positioning techniques, it’s possible that our system will recognize an ad unit as the first ad unit, even if it appears to the end user at the very bottom of the page.

Make sure the ad unit with the highest clickthrough rate is the first instance of the ad code that appears in the HTML. Since the first ad unit is always filled before the rest, you want to make sure that ad unit is located in the best placement on your page.