There's no single solution when it comes to positioning your AdSense ads. The best place to position your AdSense ads is where the visitor's attention is focused, and visitors interact with every site differently, depending on the type of site and how the navigation and features is arranged.
In general, there are some areas in a web page that visitor's eyes and attention is focused to more than the other. By compiling and analyzing the data of visitor's behaviour, Google itself came up with the following “heat map”.
While the heat map above was provided by Google for AdSense unit placements, it's also applicable to any other PPC or affiliate ads. The heat map suggest that the darker the area, more likely it is for visitors to look at your ads, and the better your ads will perform.
From the diagram we can tell that the best performing ads are normally the one placed just before the main content of the site, and followed by other “above the fold” locations and the left sidebar.
Do keep in mind though, though this is how it generally works for most websites, it might work differently for you. The general rule is, place it where your site's visitor's are focused to, and that could always mean near your site's content; which could be some widgets you have, article body, Youtube video, so on and so forth. Whatever it is, keep experimenting till you find the right spot. Also keep in mind to not overdo with your ad placement, and try to keep your site clean, uncluttered, and inviting.