Depends on time of year, but
I would second the idea of the Netherlands - and many places away from the more populated western parts, e.g. Friesland and islands in the north, parts of eastern middle or south near Eindhoven. Potentially also Estonia (flat terrain and my travels suggested potential network of routes).
In Australia, I would think of Freemantle, Rottnest Island and area south of Perth.
In the US, Fredericksburg, TX is a beautiful place in Spring, some hills but also less hilly routes.