Me too, because of the inventory, but 728x90 is still my star.

Right, no slight on 728x90, still my top performer, too! I experimented with those and the medium rectangles as the second and third ad units and the rectangles did better as supporting players.

Neither of you use the 336x280 adblock? I like it because it will show 336x280 or 300x250, whichever they think will pay better, as I understand it.

I see this in action all the time on my website. The visual difference is that a 300x250 will top and centered in a 336x280 space. So you get a little more white space on the sides and bottom, not enough to be distracting though.

I don't use any specifically sized ad blocks because everything is responsive and Smart Sized. Having said that, I often restrict the containing div's width with css as well as specify ad orientations to roughly achieve what ken_b is describing. While not specifying specific sizes, the approach does allow you to be able to anticipate what sizes can possibly show up in the slot.

I don't use any ad blocks in with the content; they're always at the top, in the footer and/or in the sidebars. I use the 300x250 because it's a more standard size and I don't want to resize my main content area to allow for a larger sidebar.