Many sites have their login and navigation systems on the top right. So putting ads here can be quite effective if people are not blind to this area of the website.
There are actually arguments contradicting this placement of ad in the top right, I read it somewhere here, unfortunately I forgot where on this forum it was
Anyway, it really depends on your site design and functionality on where the best placement is. The worst place I suppose, would be on the top right if there is a left column as well.