The large rectangle and the medium rectange both have been performing about the same. I have to leave the same format for a week or two before it optimizes. I also make good use of the "ad targeting" feature for advertisers.
Well, this poll is in wrong category. It should be in one of the sub-categories of AdSense forum. Whatever, But, I will go for Large Rectangle and A Banner
It should be considered also as where your ad placement does fit in the bigger theme of your site but I go with large rectangle.
The 336x280 large rectange is still performing the best, although the 200x90 Link Unit has been performing nearly as well believe it or not.
it all depends upon content sometime even 200 x 200 Small Square embedded in your content can give you good CTR