The flattest place I've ever been was East Colorado.
Weld county and east can be pretty flat, but the flattest place I have been is the Red River Valley of the North in Eastern North Dakota/Western Minnesota.
The area was a lake during the Ice Age (Glacial Lake Aggasiz) and the lake bottom is the current ground surface. It is flatter than the open ocean, because the ocean conforms to the general curvature of the planet. IIRC early microwave transmission experiments were carried out there because the line of sight there is one of the longest on the planet.
The Utah Salt Flats rate pretty flat, too.
Apparently, though, Salar de Uyuni in Bolivia (a salt flat) is the flattest place on Earth.