The answer is:  It depends on your site content and your target audience.

Make sure you test all of the available AdSense ad types and placements on your pages.  Let each of them run for a while, add up the results, and move on to the next test.  After you've tested all of the ad types and placements, you simply choose the one that works best for your particular site or high-traffic page.  I often see people slap up their ads and then never change the positioning or ad type - this is a huge mistake.  Change things up to increase your income.

My testing has revealed that one ad type and placement usually performs a little better than the rest on my sites - just make sure you try them all...