The question about the best Adsense unit is asked many times in webmaster forums each day and users will definitely get different answers depending on the person and his websites. Certain trends suggest that the medium and large rectangle are the best performing ad units when placed next to content while the leaderboard ad performs best in places where users would look for navigation. The problem I have with leaderboards is that they take away the most expensive ads if it is placed on top which means that the rectangle units near the content will earn less.