This series stands out from the pack. I am in the midst of watching the whole thing for the second time, and I admire it even more now. The low-key but growing depth of the main characters is impressive especially because it is so subtle. Tosh begins as a comical figure and grows into a deeply lovable one. Sandy is endearing and decent almost to a fault, but has his relatable problems. Cora and Billy are always welcome appearances. I am less enamored of the daughter and her annoying non-father, but that is a mild complaint compared to my admiration for Douglas Henshall and his deeply intelligent, still-waters-run-deep, compelling interpretations of the main character. And then there's the mystique of the Shetland Islands, which are main characters in themselves! Listen to the accents long enough and you won't be able to pronounce "Shetland" in the American way again.
Of all the British offerings, this one is close to the head of the list...and I have watched a great many of them.