Probably top middle, but you can find it for your self. Just place it for a few days on a different positions.
I have gone through google case studies on ad positioning. My weight website and blog use 160x sky scrappers T and top 460 X banners that are doing pretty well.. Read the google case study in google adsense forum. That will help you
for my experience, top right 160*600 and top center 728*90 without borders and with background like site background
Place the ads where the visitors look at the most like 125x125 small button over the left or right top corner simply above the menu followed by menu and then 160x600 ad format on the side bar. The 336x280 or 300x250 ad format merged with the contents in the body. Blending the ads with contents will help improve the CTR
Their are many but you have to check which will work better in your web design . You can share your design and we can help you . Check top adsense earners site for inspiration .
For me, the best position should be in between blog posts and content. Another one is the skyscraper on the sidebar.
For every site, the best ad placement depends on many things, mostly on the design of the site. Can you show us your site?