For Greece, the least visited region by tourists is Western Macedonia, here you get an idea. You see, this region is the only region in Greece where it is landlocked. Since Greece is known for it's beaches and being sunny, you can imagine why this region doesn't get many tourists.
Now don't get me wrong the region does have beautiful landscapes and forests, but that isn't something most people would think at first when thinking about Greece.
Now there are some cities you can visit such as Kozani, Grevena, Kastoria and Florina, but these cities aren't generally known to tourists anyway.