Any county in Iowa could work here, what I would be looking for though if I was starting with a blank slate is...ISOLATION. 100 acres of cover surrounded by LOTS of Ag would be my goal. Any scenario that allows a buck to get to 5+ years old will produce giants here, IMO. But, as others have said, even really good areas/counties can be limited by neighbor behavior.
An otherwise good area, with good cover on multiple properties with good hunters who really can't lay off of hi po 3 and 4 year olds will likely be frustrating to the one dude who passes up the hi po young bucks when others around them don't.