What State Is The Flattest?

Florida:

Once they’d developed their algorithm, Dobson and Campbell processed elevation data, gathered from NASA’s Shuttle Radar Topography Mission, for the contiguous U.S.—48 states and the District of Columbia. (Why not Alaska and Hawaii? “We knew the answer for those well enough,” Dobson told me; “we knew they weren't going to be the flattest.”) They gridded their findings into 90-meter cell. Then they categorized each cell as not flat, flat, flatter, or flattest. (“It was a lot of computation,” Dobson says, “because we're measuring every 90 meters, or every 300 feet, across the country.”)
From there, they compared each state’s percentage of flatness—and ranked them. The top 10 flattest states, per their results (and ranked according to their total flat, flatter, and flattest designations):

Florida
Illinois
North Dakota
Louisiana
Minnesota
Delaware
Kansas

Texas

Nevada
Indiana

In other words, Dobson says: “What we're seeing is the percentage of Kansas that is flat is a lower percentage than Florida.”
While the list offers a ranked percentage of state area in the flat, flatter, andflattest categories, you can see a rank of states according to the flattest category alone charted on the map above.

All of this grew out of a study which tried to determine if Kansas was flatter than a pancake (it was).  Eastern Ohio keeps us from coming anywhere near the top ten.  Not that we really need to break into it.  If I had to pick though, I'd take some of that flat, black Illinois dirt.