This entirely depends on your layout. The goal is to get more impressions and clicks, so you want to place your ads in a more prominent spot. We have found that ads within post content toward the top of the screen tend to get the most clicks.
Several users have asked us about adding AdSense in a floating widget because it will be very prominent. We strongly recommend against that because it is not allowed by Google Terms of Service. If you violate their TOS, then you can get banned from the program.