Why wouldn't you get the domain with the most impressions and then work to improve the click-thru rate? (other than Joel's great point about Google controlling impressions)
Because you have no reason to assume that the one with the most impressions is the top performing ads.
Because of the way impression share is distributed (and because we don't know what ad rotation setting OP had on), the top impression ad might only have the most impressions because Google decided to show it more rather than it actually being superior on any actual performance metric.
